Evaluate generator expressions in generated pkgconfig file (#1232)

Summary:
The variables written to `libfolly.pc` by CMake may contain generator
expressions, in which case `libfolly.pc` ends up containing them
verbatim.  This adds a postprocessing stage in which the variables are
evaluated to yield the final `libfolly.pc`.
Pull Request resolved: https://github.com/facebook/folly/pull/1232

Reviewed By: simpkins

Differential Revision: D17637013

Pulled By: yfeldblum

fbshipit-source-id: d1cc2d3322fe7b15bfff4a99a246dcc6e267a0ba

CMakeLists.txt

@@ -397,9 +397,14 @@ install(
 # CMake can depend on folly using pkg-config.
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/CMake/libfolly.pc.in
-  ${CMAKE_CURRENT_BINARY_DIR}/libfolly.pc
+  ${CMAKE_CURRENT_BINARY_DIR}/libfolly.pc.gen
   @ONLY
 )
+file(
+  GENERATE
+  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/libfolly.pc
+  INPUT ${CMAKE_CURRENT_BINARY_DIR}/libfolly.pc.gen
+)
 install(
   FILES ${CMAKE_CURRENT_BINARY_DIR}/libfolly.pc
   DESTINATION ${LIB_INSTALL_DIR}/pkgconfig