encode PACKAGE_VERSION in libfolly.pc

Summary:
`libfolly.pc` should contain the actual version of Folly, instead of just
`master`.

Reviewed By: danobi

Differential Revision: D24471074

fbshipit-source-id: 5fc29eeee25c8c317c3405f430fd079d47c441bd

CMake/libfolly.pc.in

@@ -5,7 +5,7 @@ includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 
 Name: libfolly
 Description: Facebook (Folly) C++ library
-Version: master
+Version: @PACKAGE_VERSION@
 Cflags: -I${includedir} @FOLLY_PKGCONFIG_CFLAGS@
 Libs: -L${libdir} -lfolly
 Libs.private: @FOLLY_PKGCONFIG_PRIVATE_LIBS@