Tell configure about gmp (#5585)

This is a partial fix for #5564.

This package used to trust that `configure` would discover `gmp` from
its environment.

It's safer to tell it where to find `gmp` explicitly.

This does that by adding a configure_args() that provides a
`--with-gmp=...` argument for configure.

var/spack/repos/builtin/packages/mpfr/package.py

@@ -42,3 +42,9 @@ class Mpfr(AutotoolsPackage):
 
     patch('vasprintf.patch', when='@3.1.5')
     patch('strtofr.patch',   when='@3.1.5')
+
+    def configure_args(self):
+        args = [
+            '--with-gmp=' + self.spec['gmp'].prefix,
+        ]
+        return args