remove usage of 'c_plusplus' preprocessor macro

Use only __cplusplus which is supported by the C++ standard.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/xf86drm.h b/xf86drm.h
index e3a19dc..360e04a 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -39,7 +39,7 @@
 #include <stdint.h>
 #include <drm.h>
 
-#if defined(__cplusplus) || defined(c_plusplus)
+#if defined(__cplusplus)
 extern "C" {
 #endif
 
@@ -759,7 +759,7 @@
 extern char *drmGetPrimaryDeviceNameFromFd(int fd);
 extern char *drmGetRenderDeviceNameFromFd(int fd);
 
-#if defined(__cplusplus) || defined(c_plusplus)
+#if defined(__cplusplus)
 }
 #endif