bpo-32233: Fix build with --with-system-libmpdec. (#4739)

setup.py

@@ -2069,6 +2069,7 @@ class PyBuildExt(build_ext):
                                                          'Modules',
                                                          '_decimal',
                                                          'libmpdec'))]
+            libraries = ['m']
             sources = [
               '_decimal/_decimal.c',
               '_decimal/libmpdec/basearith.c',
@@ -2164,7 +2165,7 @@ class PyBuildExt(build_ext):
         ext = Extension (
             '_decimal',
             include_dirs=include_dirs,
-            libraries=['m'],
+            libraries=libraries,
             define_macros=define_macros,
             undef_macros=undef_macros,
             extra_compile_args=extra_compile_args,