OpenJDK uses -xc99=%none but /usr/include/X11/Xfuncproto.h contains a C99-only feature:

132 /* requires xproto >= 7.0.22 */
133 #if defined(__GNUC__) && ((__GNUC__ * 100 + __GNUC_MINOR__) >= 303)
134 #define _X_NONNULL(args...) __attribute__((nonnull(args)))
135 #else
136 #define _X_NONNULL(...) /* */
137 #endif

When building OpenJDK, it fails with

"/usr/include/X11/Xfuncproto.h", line 136: invalid token in #define
macro parameters: ...
cc: acomp failed for
../../../src/share/native/sun/awt/image/BufImgSurfaceData.c