gh-1019: Fix FreeBSD 10 build issue with gcc4.8

Issue was due to _Thread_local keyword used in the system
header which gcc4.8 fails to understand in c11 mode.
In --std=gnu99 mode it works.

Updated cmake test to claim no c11 support if ctype.h fails
to compile.

cmake/compiler.cmake

@@ -41,7 +41,14 @@ endif()
 if((NOT HAVE_STD_C11 AND NOT HAVE_STD_GNU99) OR
    (NOT HAVE_STD_CXX11 AND NOT HAVE_STD_GNUXX0X))
     set(CMAKE_REQUIRED_FLAGS "-std=c11")
-    check_c_source_compiles("int main(void) { return 0; }" HAVE_STD_C11)
+    check_c_source_compiles("
+    /*
+     * FreeBSD 10 ctype.h header fail to compile on gcc4.8 in c11 mode.
+     * Make sure we aren't affected.
+     */
+    #include <ctype.h>
+    int main(void) { return 0; }
+    " HAVE_STD_C11)
     set(CMAKE_REQUIRED_FLAGS "-std=gnu99")
     check_c_source_compiles("int main(void) { return 0; }" HAVE_STD_GNU99)
     set(CMAKE_REQUIRED_FLAGS "-std=c++11")