Improve llvm libc++ detection

The old detection did only work if CFLAGS contained -std=c++11, since ciso646
was only included for __cplusplus > 199711.
Thanks to Koernel for the cmake part.

config/lyxinclude.m4

@@ -185,6 +185,10 @@ AC_LANG_POP(C++)
 
 if test $lyx_cv_lib_stdcxx = "yes" ; then
   AC_DEFINE(STD_STRING_USES_COW, 1, [std::string uses copy-on-write])
+else
+  if test $lyx_cv_prog_clang = "yes" ; then
+    AC_DEFINE(USE_LLVM_LIBCPP, 1, [use libc++ provided by llvm instead of GNU libstdc++])
+  fi
 fi
 
 ### We might want to get or shut warnings.

development/cmake/ConfigureChecks.cmake

@@ -139,16 +139,24 @@ check_cxx_source_compiles(
 	"
 STD_STRING_USES_COW)
 
-#get_filename_component(cxx_base "${CMAKE_CXX_COMPILER}" NAME_WE)
-#if(cxx_base MATCHES "^clang(-[0-9]+\\.[0-9]+\\.[0-9]+)?(\\+\\+)?$")
-#	set(STD_STRING_USES_COW)
-#else()
-#	if("${MINGW}")
-#		set(STD_STRING_USES_COW)
-#	else()
-#		set(STD_STRING_USES_COW 1)
-#	endif()
-#endif()
+check_cxx_source_compiles(
+	"
+	#ifndef __clang__
+		this is not clang
+	#endif
+	int main() {
+	  return(0);
+	}
+	"
+lyx_cv_prog_clang)
+
+set(USE_LLVM_LIBCPP)
+if(NOT STD_STRING_USES_COW)
+  if(lyx_cv_prog_clang)
+    # use libc++ provided by llvm instead of GNU libstdc++
+    set(USE_LLVM_LIBCPP 1)
+  endif()
+endif()
 
 if(LYX_USE_QT MATCHES "QT5")
   if (Qt5X11Extras_FOUND)

development/cmake/config.h.cmake

@@ -63,6 +63,9 @@
 // Define if std::string uses copy-on-write
 #cmakedefine STD_STRING_USES_COW 1
 
+// use libc++ provided by llvm instead of GNU libstdc++
+#cmakedefine USE_LLVM_LIBCPP 1
+
 #cmakedefine Z_PREFIX 1
 
 ${Include_used_spellchecker}

src/support/debug.h

@@ -20,7 +20,7 @@
 // Forward definitions do not work with libc++
 // but ios_base has already been defined in strfwd
 // if compiling with it
-#ifndef  _LIBCPP_VERSION
+#ifndef USE_LLVM_LIBCPP
 namespace std {
 
 class ios_base;

src/support/strfwd.h

@@ -13,14 +13,6 @@
 #ifndef STRFWD_H
 #define STRFWD_H
 
-// This includes does nothing but defining _LIBCPP_VERSION
-// if libc++ is used (rather than libstdc++) - we first
-// check if we have at least a c++03 standard before
-// including the file
-#if (__cplusplus > 199711L)
-#include <ciso646>
-#endif
-
 #ifdef USE_WCHAR_T
 
 // Prefer this if possible because GNU libstdc++ has usable
@@ -37,7 +29,7 @@ namespace lyx { typedef boost::uint32_t char_type; }
 #endif
 
 // Forward definitions do not work with libc++
-#ifdef  _LIBCPP_VERSION
+#ifdef USE_LLVM_LIBCPP
 #include <string>
 #else