Workaround broken flag detection on GCC

Author: ldionne

libcxx/cmake/config-ix.cmake

@@ -135,5 +135,7 @@ else()
   check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
   check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
   check_library_exists(atomic __atomic_fetch_add_8 "" LIBCXX_HAS_ATOMIC_LIB)
-  check_library_exists(m lgamma_r "" LIBCXX_HAS_MATH_LIB)
+  if (CMAKE_SYSTEM_NAME STREQUAL "Linux" AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    set(LIBCXX_HAS_MATH_LIB ON)
+  endif()
 endif()