Add HERMES_IS_MOBILE_BUILD flag.

Summary:
Add a `HERMES_IS_MOBILE_BUILD` flag which allows us to exclude
features by default on phones while keeping them around on non-mobile
builds.

This is useful for sections of code in the compiler which increase code
size and are only used on the compiling server, such as the ability to
parse Flow types.

These new features can be gated by default under `HERMES_IS_MOBILE_BUILD`,
allowing simple defaults.

Reviewed By: tmikov

Differential Revision: D20290497

fbshipit-source-id: 1d310bf86e26076e481353a9fd148af89b20cbc4

CMakeLists.txt

@@ -144,6 +144,9 @@ set(HERMESVM_JIT_DISASSEMBLER OFF CACHE BOOL
 set(HERMES_USE_FLOWPARSER OFF CACHE BOOL
   "Use libflowparser for parsing es6")
 
+set(HERMES_IS_MOBILE_BUILD OFF CACHE BOOL
+  "Building for a mobile device")
+
 set(HERMES_IS_ANDROID OFF CACHE BOOL
   "Building for Android")
 
@@ -167,6 +170,8 @@ set(EMSCRIPTEN_FASTCOMP ON CACHE BOOL
   "Emscripten is using the fastcomp backend instead of the LLVM one")
 
 if (HERMES_IS_ANDROID)
+  set(HERMES_IS_MOBILE_BUILD TRUE)
+
   add_definitions(-DHERMES_PLATFORM_UNICODE=HERMES_PLATFORM_UNICODE_JAVA)
 
   # The toolchain passes -Wa,--noexecstack which is valid for compiling
@@ -187,6 +192,10 @@ if (HERMES_IS_ANDROID)
   endif()
 endif()
 
+if (HERMES_IS_MOBILE_BUILD)
+  add_definitions(-DHERMES_IS_MOBILE_BUILD)
+endif()
+
 # Enable debug mode by default
 if ((NOT GENERATOR_IS_MULTI_CONFIG) AND CMAKE_BUILD_TYPE STREQUAL "")
     set(CMAKE_BUILD_TYPE Debug)