clang: suppress a warning

Define missing _GNU_SOURCE for mkostemp().

    lib/ii.c:7217:30: warning: implicit declaration
          of function 'mkostemp' is invalid in C99 [-Wimplicit-function-declaration]
              ii_buffer->tmpfd = GRN_MKOSTEMP(ii_buffer->tmpfpath,
                                 ^
    lib/groonga_in.h:523:45: note: expanded from
          macro 'GRN_MKOSTEMP'
    #  define GRN_MKOSTEMP(template,flags,mode) mkostemp(template,flags)
                                                ^

CMakeLists.txt

@@ -91,6 +91,10 @@ add_definitions(
   -DHAVE_CONFIG_H
   )
 
+if(CMAKE_COMPILER_IS_GNUC OR CMAKE_C_COMPILER_ID STREQUAL "Clang")
+  set(_GNU_SOURCE TRUE)
+endif()
+
 include_directories(
   BEFORE
   ${CMAKE_CURRENT_BINARY_DIR}

config.h.cmake

@@ -58,7 +58,9 @@
 
 /* compiler specific build options */
 #cmakedefine _FILE_OFFSET_BITS @_FILE_OFFSET_BITS@
-#cmakedefine _GNU_SOURCE
+#ifndef _GNU_SOURCE
+ #cmakedefine _GNU_SOURCE
+#endif
 #cmakedefine _ISOC99_SOURCE
 #cmakedefine _LARGE_FILES
 #cmakedefine _NETBSD_SOURCE