nettle: fix broken build (#11611)

* nettle: force C99 build

* nettle: update to use c99_flag

from @alalazo

Co-Authored-By: Massimiliano Culpo <massimiliano.culpo@gmail.com>

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

@@ -22,3 +22,6 @@ class Nettle(AutotoolsPackage):
 
     depends_on('gmp')
     depends_on('m4', type='build')
+
+    def configure_args(self):
+        return ['CFLAGS={0}'.format(self.compiler.c99_flag)]