Revert "[CMake] Use __libc_start_main rather than fopen when checking…

… for C library"

This reverts commit r352341: it broke the build on macOS which doesn't
seem to provide __libc_start_main in its C library.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@352411 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -2,7 +2,7 @@ include(CheckLibraryExists)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
-check_library_exists(c __libc_start_main "" LIBCXXABI_HAS_C_LIB)
+check_library_exists(c fopen "" LIBCXXABI_HAS_C_LIB)
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXXABI_HAS_GCC_S_LIB)
 endif ()