Make sure that even in the standalone build configuration, we can bui…

…ld doxygen documentation.

The issue is that this was originally done when in-tree builds were the
preferred way of building so LLVM_ENABLE_DOXYGEN would be defined and LLVM would
have found doxygen as well. When one is doing the current preferred standalone
install, the option LLVM_ENABLE_DOXYGEN is not discoverable to the user via an
option with a default argument and the doxygen package is never searched for.

This commit ensures that when building standalone:

1. LLVM_ENABLE_DOXYGEN is defined as an option with a default value of FALSE.
2. The cmake dtrace package is searched for and found.

When we are building standalone, we take these values from LLVM.

CMakeLists.txt

@@ -606,6 +606,10 @@ if(LIBXML2_FOUND)
   set(SWIFT_HAVE_LIBXML 1)
 endif()
 
+if (LLVM_ENABLE_DOXYGEN)
+  message(STATUS "Doxygen: enabled")
+endif()
+
 #
 # Set up global CMake variables for API notes.
 #

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -261,6 +261,11 @@ macro(swift_common_standalone_build_config product is_cross_compiling)
 
   set(LLVM_INCLUDE_TESTS TRUE)
   set(LLVM_INCLUDE_DOCS TRUE)
+
+  option(LLVM_ENABLE_DOXYGEN "Enable doxygen support" FALSE)
+  if (LLVM_ENABLE_DOXYGEN)
+    find_package(Doxygen REQUIRED)
+  endif()
 endmacro()
 
 # Common cmake project config for unified builds.