[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

llvm-svn: 352341

compiler-rt/cmake/config-ix.cmake

@@ -12,7 +12,7 @@ function(check_linker_flag flag out_var)
   cmake_pop_check_state()
 endfunction()
 
-check_library_exists(c fopen "" COMPILER_RT_HAS_LIBC)
+check_library_exists(c __libc_start_main "" COMPILER_RT_HAS_LIBC)
 if (COMPILER_RT_USE_BUILTINS_LIBRARY)
   include(HandleCompilerRT)
   find_compiler_rt_library(builtins COMPILER_RT_BUILTINS_LIBRARY)

libcxx/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 fopen "" LIBCXX_HAS_C_LIB)
+  check_library_exists(c __libc_start_main "" LIBCXX_HAS_C_LIB)
 endif()
 
 if (NOT LIBCXX_USE_COMPILER_RT)

libcxxabi/cmake/config-ix.cmake

@@ -2,7 +2,7 @@ include(CheckLibraryExists)
 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 ()

libunwind/cmake/config-ix.cmake

@@ -3,7 +3,7 @@ include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 include(CheckLibraryExists)
 
-check_library_exists(c fopen "" LIBUNWIND_HAS_C_LIB)
+check_library_exists(c __libc_start_main "" LIBUNWIND_HAS_C_LIB)
 
 if (NOT LIBUNWIND_USE_COMPILER_RT)
   check_library_exists(gcc_s __gcc_personality_v0 "" LIBUNWIND_HAS_GCC_S_LIB)

llvm/runtimes/CMakeLists.txt

@@ -98,7 +98,7 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_CURRENT_SOURCE_DIR})
   include(CheckLibraryExists)
   include(CheckCCompilerFlag)
 
-  check_library_exists(c fopen "" LLVM_HAS_C_LIB)
+  check_library_exists(c __libc_start_main "" LLVM_HAS_C_LIB)
   check_c_compiler_flag(-nodefaultlibs LLVM_HAS_NODEFAULTLIBS_FLAG)
   if(LLVM_HAS_NODEFAULTLIBS_FLAG)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nodefaultlibs")