gdbm: Get working on CentOS5 and Other Older Systems (#8134)

* Fix #7691

* flake8

* code review

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

@@ -46,4 +46,12 @@ class Gdbm(AutotoolsPackage):
     depends_on("readline")
 
     def configure_args(self):
-        return ['--enable-libgdbm-compat']
+
+        # GDBM uses some non-standard GNU extensions,
+        # enabled with -D_GNU_SOURCE.  See:
+        #   https://patchwork.ozlabs.org/patch/771300/
+        #   https://stackoverflow.com/questions/5582211
+        #   https://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html
+        return [
+            '--enable-libgdbm-compat',
+            'CPPFLAGS=-D_GNU_SOURCE']