Make GFlags actually optional (#1325)

Summary:
The absence of GFlags is already partially optional -
portability/GFlags.h defines a number of stub macros / clases if
FOLLY_HAVE_LIBGFLAGS is false.

However the top-level CMake rules don't correctly handle when GFlags
isn't present - they unconditionally add GFlags-related variables to
CMAKE_REQUIRED_xxx variables.

Additionally there are a couple of source files which should not be
compiled if GFlags isn't available.
Pull Request resolved: https://github.com/facebook/folly/pull/1325

Differential Revision: D20252030

Pulled By: yfeldblum

fbshipit-source-id: 7c3441e8b2850d68df125fb9c14b8417f4d08b55

CMake/folly-deps.cmake

@@ -38,10 +38,12 @@ list(APPEND FOLLY_INCLUDE_DIRECTORIES ${DOUBLE_CONVERSION_INCLUDE_DIR})
 
 find_package(Gflags MODULE)
 set(FOLLY_HAVE_LIBGFLAGS ${LIBGFLAGS_FOUND})
-list(APPEND FOLLY_LINK_LIBRARIES ${LIBGFLAGS_LIBRARY})
-list(APPEND FOLLY_INCLUDE_DIRECTORIES ${LIBGFLAGS_INCLUDE_DIR})
-list(APPEND CMAKE_REQUIRED_LIBRARIES ${LIBGFLAGS_LIBRARY})
-list(APPEND CMAKE_REQUIRED_INCLUDES ${LIBGFLAGS_INCLUDE_DIR})
+if(LIBGFLAGS_FOUND)
+  list(APPEND FOLLY_LINK_LIBRARIES ${LIBGFLAGS_LIBRARY})
+  list(APPEND FOLLY_INCLUDE_DIRECTORIES ${LIBGFLAGS_INCLUDE_DIR})
+  list(APPEND CMAKE_REQUIRED_LIBRARIES ${LIBGFLAGS_LIBRARY})
+  list(APPEND CMAKE_REQUIRED_INCLUDES ${LIBGFLAGS_INCLUDE_DIR})
+endif()
 
 find_package(Glog MODULE)
 set(FOLLY_HAVE_LIBGLOG ${GLOG_FOUND})

CMakeLists.txt

@@ -272,6 +272,16 @@ else()
     ${FOLLY_DIR}/experimental/crypto/LtHash.h
   )
 endif()
+if (NOT ${LIBGFLAGS_FOUND})
+  list(REMOVE_ITEM files
+    ${FOLLY_DIR}/experimental/NestedCommandLineApp.cpp
+    ${FOLLY_DIR}/experimental/ProgramOptions.cpp
+  )
+  list(REMOVE_ITEM hfiles
+    ${FOLLY_DIR}/experimental/NestedCommandLineApp.h
+    ${FOLLY_DIR}/experimental/ProgramOptions.h
+  )
+endif()
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
   list(REMOVE_ITEM files
     ${FOLLY_DIR}/Poly.cpp