libc++: Perform configuration checks with -nodefaultlibs

We're compiling libc++ with -nodefaultlibs, so we should also pass this option
during the configuration checks to ensure those checks are consistent with the
actual build.

The primary motivation here is to ease cross-compilation against a non-standard
set of C++ libraries. Previously, the configuration checks would attempt to link
against the standard C++ libraries, which would cause link failures when
cross-compiling, even though the actual library link would go through correctly
(because of the use of -nodefaultlibs and explicitly specifying any needed
libraries). This is more correct even ignoring the motivation, however.

Patch by Shoaib Meenai!

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279584 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/config-ix.cmake

@@ -1,5 +1,26 @@
 include(CheckLibraryExists)
 include(CheckCXXCompilerFlag)
+
+check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
+check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)
+
+# libc++ is built with -nodefaultlibs, so we want all our checks to also
+# use this option, otherwise we may end up with an inconsistency between
+# the flags we think we require during configuration (if the checks are
+# performed without -nodefaultlibs) and the flags that are actually
+# required during compilation (which has the -nodefaultlibs). libc is
+# required for the link to go through.
+check_cxx_compiler_flag(-nodefaultlibs LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+if (LIBCXX_SUPPORTS_NODEFAULTLIBS_FLAG)
+  list(APPEND CMAKE_REQUIRED_LIBRARIES -nodefaultlibs)
+  if (LIBCXX_HAS_C_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES c)
+  endif ()
+  if (LIBCXX_HAS_GCC_S_LIB)
+    list(APPEND CMAKE_REQUIRED_LIBRARIES gcc_s)
+  endif ()
+endif ()
+
 include(CheckLibcxxAtomic)
 
 # Check compiler flags
@@ -14,7 +35,5 @@ check_cxx_compiler_flag(/GR-                    LIBCXX_HAS_NO_GR_FLAG)
 
 # Check libraries
 check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
-check_library_exists(c fopen "" LIBCXX_HAS_C_LIB)
 check_library_exists(m ccos "" LIBCXX_HAS_M_LIB)
 check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)
-check_library_exists(gcc_s __gcc_personality_v0 "" LIBCXX_HAS_GCC_S_LIB)