avoid _Static_assert from c++

Jonathan Perkin applied the following changes to pkgsrc with the commit message:

libXt: Do not use _Static_assert in C++ code.                                                                                                  
                                                                                                                                               
Fixes lots of dependencies.

I'm currently lost in gitlab and can't find the 'fork' button, so here are his changes in-line:

diff --git a/include/X11/Intrinsic.h b/include/X11/Intrinsic.h
index 71199e7..94197f9 100644
--- a/include/X11/Intrinsic.h
+++ b/include/X11/Intrinsic.h
@@ -174,7 +174,7 @@ typedef unsigned short      Dimension;  /* Size in pixels                   */
 typedef short          Position;   /* Offset from 0 coordinate         */
 
 typedef void*          XtPointer;
-#if __STDC_VERSION__ >= 201112L
+#if __STDC_VERSION__ >= 201112L && !defined(__cplusplus)
 _Static_assert(sizeof(XtArgVal) >= sizeof(XtPointer), "XtArgVal too small");
 _Static_assert(sizeof(XtArgVal) >= sizeof(long), "XtArgVal too small");
 #endif
diff --git a/include/X11/IntrinsicP.h b/include/X11/IntrinsicP.h
index 21c5aca..a2d8a29 100644
--- a/include/X11/IntrinsicP.h
+++ b/include/X11/IntrinsicP.h
@@ -64,7 +64,7 @@ typedef struct {
     XtIntPtr   xrm_default_type; /* Default representation type quark  */
     XtPointer  xrm_default_addr; /* Default resource address           */
 } XrmResource, *XrmResourceList;
-#if __STDC_VERSION__ >= 201112L
+#if __STDC_VERSION__ >= 201112L && !defined(__cplusplus)
 _Static_assert(XtOffsetOf(XrmResource, xrm_default_addr) ==
                    XtOffsetOf(XtResource, default_addr),
                "Field offset mismatch");