add shared library support to add_fbthrift_cpp_library

Summary:
`add_fbthrift_cpp_library` should honor `BUILD_SHARED_LIBS`, and call
`add_library` with the right setting (`SHARED` if enabled, `STATIC` otherwise)

Reviewed By: yns88

Differential Revision: D24911124

fbshipit-source-id: 79df7640a758a592a3df3e9e79bb129dd57f2d47

Author: michel-slm

build/fbcode_builder/CMake/FBThriftCppLibrary.cmake

@@ -127,8 +127,14 @@ function(add_fbthrift_cpp_library LIB_NAME THRIFT_FILE)
   )
 
   # Now emit the library rule to compile the sources
+  if (BUILD_SHARED_LIBS)
+    set(LIB_TYPE SHARED)
+  else ()
+    set(LIB_TYPE STATIC)
+  endif ()
+
   add_library(
-    "${LIB_NAME}" STATIC
+    "${LIB_NAME}" ${LIB_TYPE}
     ${generated_sources}
   )