Replace HAVE_* tests with just checking for (__solaris__)

The installed headers can't be relying on the presence of the internal
config.h defines, and it was breaking the xcb build for me as well due to
config.h not being included early enough.

src/xcb.h

@@ -29,10 +29,10 @@
 #define __XCB_H__
 #include <sys/types.h>
 
-#if HAVE_STDINT_H
-#include <stdint.h>
-#elif HAVE_INTTYPES_H
+#if defined(__solaris__)
 #include <inttypes.h>
+#else
+#include <stdint.h>
 #endif
 
 /* FIXME: these names conflict with those defined in Xmd.h. */