Avoid using flags not supported by in-tree gcc.

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -36,7 +36,9 @@ if (NOT APPLE)
     add_compile_options($<$<CONFIG:Debug>:-Wsystem-headers>)
 endif()
 add_compile_options($<$<CONFIG:Release>:-Wuninitialized>)
+if ("${CMAKE_C_COMPILER_ID}" STREQUAL "Clang")
 add_compile_options($<$<CONFIG:Release>:-Wno-dangling-else>)
+endif()
 add_compile_options(-Wno-string-compare)
 add_compile_options(-Wstack-protector -fstack-protector)
 add_compile_options(-Wstrict-aliasing -fstrict-aliasing)
