[3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8 (GH-21488)

Define UNUSED only when mpdecimal.h doesn't define it. This would support
building with system libmpdec 2.5 while retaining compatibility with system
libmpdec 2.4 or the vendored copy.

Tested to build fine with either system libmpdec or the vendored one.

Author: felixonmars

Misc/NEWS.d/next/Build/2020-07-15-17-56-32.bpo-41302.S3o-x9.rst

@@ -0,0 +1 @@
+Enable building Python 3.8 with libmpdec-2.5.0 to ease maintenance for Linux distributions. Patch by Felix Yan.

Modules/_decimal/_decimal.c

@@ -58,6 +58,13 @@
 
 #define BOUNDS_CHECK(x, MIN, MAX) x = (x < MIN || MAX < x) ? MAX : x
 
+#ifndef UNUSED
+#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
+  #define UNUSED __attribute__((unused))
+#else
+  #define UNUSED
+#endif
+#endif
 
 /* _Py_DEC_MINALLOC >= MPD_MINALLOC */
 #define _Py_DEC_MINALLOC 4