gh-108562: Fix compiler warnings for libmpdec (#114751)

If awailable, enable -fstrict-overflow for libmpdec. Also
shut off false positive warnings (-Warray-bounds).

The later was backported from mpdecimal-4.0.0.

Makefile.pre.in

@@ -1213,7 +1213,7 @@ PYTHON_HEADERS= \
 
 ##########################################################################
 # Build static libmpdec.a
-LIBMPDEC_CFLAGS=@LIBMPDEC_CFLAGS@ $(PY_STDMODULE_CFLAGS) $(CCSHARED)
+LIBMPDEC_CFLAGS=$(PY_STDMODULE_CFLAGS) @LIBMPDEC_CFLAGS@ $(CCSHARED)
 
 # "%.o: %c" is not portable
 Modules/_decimal/libmpdec/basearith.o: $(srcdir)/Modules/_decimal/libmpdec/basearith.c $(LIBMPDEC_HEADERS) $(PYTHON_HEADERS)

Modules/_decimal/libmpdec/io.c

@@ -48,6 +48,7 @@
 #if defined(__GNUC__) && !defined(__INTEL_COMPILER) && __GNUC__ >= 7
   #pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
   #pragma GCC diagnostic ignored "-Wmisleading-indentation"
+  #pragma GCC diagnostic ignored "-Warray-bounds"
 #endif
 
 

configure.ac

@@ -3896,6 +3896,11 @@ AS_VAR_IF([with_system_libmpdec], [yes], [
   LIBMPDEC_LDFLAGS="-lm \$(LIBMPDEC_A)"
   LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
 
+  dnl Enable strict-overflow for libmpdec, if available, see GH-108562
+  AS_VAR_IF([ac_cv_cc_supports_fstrict_overflow], [yes], [
+    AS_VAR_APPEND([LIBMPDEC_CFLAGS], [" -fstrict-overflow"])
+  ])
+
   dnl Disable forced inlining in debug builds, see GH-94847
   AS_VAR_IF([with_pydebug], [yes], [
     AS_VAR_APPEND([LIBMPDEC_CFLAGS], [" -DTEST_COVERAGE"])