[CMake] Use __libc_start_main rather than fopen when checking for C library

The check_library_exists CMake uses a custom symbol definition. This
is a problem when checking for C library symbols because Clang
recognizes many of them as builtins, and returns the
-Wbuiltin-requires-header (or -Wincompatible-library-redeclaration)
error. When building with -Werror which is the default, this causes
the check_library_exists check fail making the build think that C
library isn't available.

To avoid this issue, we should use a symbol that isn't recognized by
Clang and wouldn't cause the same issue. __libc_start_main seems like
reasonable choice that fits the bill.

Differential Revision: https://reviews.llvm.org/D57142

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@352341 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

diff --git a/cmake/config-ix.cmake b/cmake/config-ix.cmake
index 379b554..4be7adc 100644
--- a/cmake/config-ix.cmake
+++ b/cmake/config-ix.cmake
@@ -2,7 +2,7 @@
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
-check_library_exists(c fopen "" LIBCXXABI_HAS_C_LIB)
+check_library_exists(c __libc_start_main "" LIBCXXABI_HAS_C_LIB)
 if (NOT LIBCXXABI_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXXABI_HAS_GCC_S_LIB)
 endif ()