Allow additional RPM macro defines (#794)

For some RPM packaging scenarios users may wish to define additional
macros.  This change enables us to do this using the `--rpmbuild_arg`
argument to the `make_rpm.py` script by passing a dict of defines to
the `pkg_rpm()` rule.

pkg/rpm_pfg.bzl

diff --git a/pkg/rpm_pfg.bzl b/pkg/rpm_pfg.bzl
index 756482b..52fc872 100644
--- a/pkg/rpm_pfg.bzl
+++ b/pkg/rpm_pfg.bzl
@@ -642,6 +642,12 @@
             "_binary_payload {}".format(ctx.attr.binary_payload_compression),
         ])
 
+    for key, value in ctx.attr.defines.items():
+        additional_rpmbuild_args.extend([
+            "--define",
+            "{} {}".format(key, value),
+        ])
+
     args.extend(["--rpmbuild_arg=" + a for a in additional_rpmbuild_args])
 
     for f in ctx.files.srcs:
@@ -1015,6 +1021,9 @@
             overcommitting your system.
             """,
         ),
+        "defines": attr.string_dict(
+            doc = """Additional definitions to pass to rpmbuild""",
+        ),
         "rpmbuild_path": attr.string(
             doc = """Path to a `rpmbuild` binary.  Deprecated in favor of the rpmbuild toolchain""",
         ),