Make build reproducible

The build is not reproducible because it embeds the build time. Embed
the date of the last modification to the source code instead (in case it
is provided via the SOURCE_DATE_EPOCH environment variable).

See the SOURCE_DATE_EPOCH specification for details:
https://reproducible-builds.org/specs/source-date-epoch/

setup.py

@@ -37,6 +37,11 @@ try:
 except ImportError:
     HAS_ZMQ = False
 
+try:
+    DATE = datetime.utcfromtimestamp(int(os.environ['SOURCE_DATE_EPOCH']))
+except (KeyError, ValueError):
+    DATE = datetime.utcnow()
+
 # Change to salt source's directory prior to running any command
 try:
     SETUP_DIRNAME = os.path.dirname(__file__)
@@ -168,7 +173,7 @@ class WriteSaltVersion(Command):
             # pylint: disable=E0602
             open(self.distribution.salt_version_hardcoded_path, 'w').write(
                 INSTALL_VERSION_TEMPLATE.format(
-                    date=datetime.utcnow(),
+                    date=DATE,
                     full_version_info=__saltstack_version__.full_info
                 )
             )
@@ -194,7 +199,7 @@ class GenerateSaltSyspaths(Command):
         # Write the system paths file
         open(self.distribution.salt_syspaths_hardcoded_path, 'w').write(
             INSTALL_SYSPATHS_TEMPLATE.format(
-                date=datetime.utcnow(),
+                date=DATE,
                 root_dir=self.distribution.salt_root_dir,
                 config_dir=self.distribution.salt_config_dir,
                 cache_dir=self.distribution.salt_cache_dir,