Fix Python detection when running build_usd.py with mayapy

The various sysconfig flags that build_usd.py relies on have
are set differently in mayapy so the code that tries to
determine the Python include and library paths failed in
various ways on different platforms. This change adds special
handling to accommodate this.

This fixes a regression that was introduced as part of the
Python 3 work in 20.05. However, we do plan to remove all
Maya-specific handling in build_usd.py in the future in
favor of code or instructions that live with the Maya
plugin itself.

(Internal change: 2055230)

build_scripts/build_usd.py

@@ -148,6 +148,18 @@ def IsVisualStudio2017OrGreater():
         return version >= VISUAL_STUDIO_2017_VERSION
     return False
 
+def IsMayaPython():
+    """Determine whether we're running in Maya's version of Python. When 
+    building against Maya's Python, there are some additional restrictions
+    on what we're able to build."""
+    try:
+        import maya
+        return True
+    except:
+        pass
+
+    return False
+
 def GetPythonInfo():
     """Returns a tuple containing the path to the Python executable, shared
     library, and include directory corresponding to the version of Python
@@ -164,30 +176,58 @@ def GetPythonInfo():
     # First we extract the information that can be uniformly dealt with across
     # the platforms:
     pythonExecPath = sys.executable
-    pythonVersion = sysconfig.get_config_var("py_version_short") 
-    pythonIncludeDir = sysconfig.get_config_var("INCLUDEPY")
+    pythonVersion = sysconfig.get_config_var("py_version_short")  # "2.7"
+    pythonVersionNoDot = sysconfig.get_config_var("py_version_nodot") # "27"
 
     # Lib path is unfortunately special for each platform and there is no
     # config_var for it. But we can deduce it for each platform, and this
     # logic works for any Python version.
-    if Windows():
-        pythonBaseDir = sysconfig.get_config_var("base")
-        pythonVersionNoDot = sysconfig.get_config_var("py_version_nodot")
-        pythonLibPath = os.path.join(pythonBaseDir, "libs",
-                                     "python" + pythonVersionNoDot + ".lib" )
-    elif Linux():
-        pythonLibDir = sysconfig.get_config_var("LIBDIR")
-        pythonLibName = sysconfig.get_config_var("LDLIBRARY")
-        pythonMultiarchSubdir = sysconfig.get_config_var("multiarchsubdir")
-        if pythonMultiarchSubdir:
-            pythonLibDir = pythonLibDir + pythonMultiarchSubdir
-        pythonLibPath = os.path.join(pythonLibDir, pythonLibName)
-    elif MacOS():
+    def _GetPythonLibraryFilename():
+        if Windows():
+            return "python" + pythonVersionNoDot + ".lib"
+        elif Linux():
+            return sysconfig.get_config_var("LDLIBRARY")
+        elif MacOS():
+            return "libpython" + pythonVersion + ".dylib"
+        else:
+            raise RuntimeError("Platform not supported")
+
+    # XXX: Handle the case where this script is being called using Maya's
+    # Python since the sysconfig variables are set up differently in Maya.
+    # Ideally we would not have any special Maya knowledge in here at all.
+    if IsMayaPython():
         pythonBaseDir = sysconfig.get_config_var("base")
+
+        # On Windows, the "base" path points to a "Python\" subdirectory
+        # that contains the DLLs for site-package modules but not the
+        # directories for the headers and .lib file we need -- those 
+        # are one level up.
+        if Windows():
+            pythonBaseDir = os.path.dirname(pythonBaseDir)
+
+        pythonIncludeDir = os.path.join(pythonBaseDir, "include",
+                                        "python" + pythonVersion)
         pythonLibPath = os.path.join(pythonBaseDir, "lib",
-                                     "libpython" + pythonVersion + ".dylib")
+                                     _GetPythonLibraryFilename())
     else:
-        raise RuntimeError("Platform not supported")
+        pythonIncludeDir = sysconfig.get_config_var("INCLUDEPY")
+        if Windows():
+            pythonBaseDir = sysconfig.get_config_var("base")
+            pythonLibPath = os.path.join(pythonBaseDir, "libs",
+                                         _GetPythonLibraryFilename())
+        elif Linux():
+            pythonLibDir = sysconfig.get_config_var("LIBDIR")
+            pythonMultiarchSubdir = sysconfig.get_config_var("multiarchsubdir")
+            if pythonMultiarchSubdir:
+                pythonLibDir = pythonLibDir + pythonMultiarchSubdir
+            pythonLibPath = os.path.join(pythonLibDir,
+                                         _GetPythonLibraryFilename())
+        elif MacOS():
+            pythonBaseDir = sysconfig.get_config_var("base")
+            pythonLibPath = os.path.join(pythonBaseDir, "lib",
+                                         _GetPythonLibraryFilename())
+        else:
+            raise RuntimeError("Platform not supported")
 
     return (pythonExecPath, pythonLibPath, pythonIncludeDir, pythonVersion)
 
@@ -1836,21 +1876,9 @@ def ForceBuildDependency(self, dep):
         PrintError("Cannot build usdview when Python support is disabled.")
         sys.exit(1)
 
-# Determine whether we're running in Maya's version of Python. When building
-# against Maya's Python, there are some additional restrictions on what we're
-# able to build.
-isMayaPython = False
-try:
-    import maya
-
-    # If the maya import succeeds and this function returns a tuple, then the
-    # USD build will be configured to use Maya's version of Python.
-    if GetPythonInfo() != None:
-        isMayaPython = True
-except:
-    pass
-
-if isMayaPython:
+# Error out if running Maya's version of Python and attempting to build
+# usdview.
+if IsMayaPython():
     if context.buildUsdview:
         PrintError("Cannot build usdview when building against Maya's version "
                    "of Python. Maya does not provide access to the 'OpenGL' "