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/libcxx/trunk@352411 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -7,7 +7,7 @@ if(WIN32 AND NOT MINGW)
   # let the default linking take care of that.
   set(LIBCXX_HAS_C_LIB NO)
 else()
-  check_library_exists(c __libc_start_main "" LIBCXX_HAS_C_LIB)
+  check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
 endif()
 
 if (NOT LIBCXX_USE_COMPILER_RT)