check for prerelease version with m4_ifset

Author: ajdavis

build/autotools/Versions.m4

@@ -1,11 +1,14 @@
 m4_define([mongoc_major_version], [1])
 m4_define([mongoc_minor_version], [1])
 m4_define([mongoc_micro_version], [5])
-
 m4_define([mongoc_prerelease_version], [dev])
 
-#m4_define([mongoc_version], [mongoc_major_version.mongoc_minor_version.mongoc_micro_version])
-m4_define([mongoc_version], [mongoc_major_version.mongoc_minor_version.mongoc_micro_version-mongoc_prerelease_version])
+m4_define(
+    [mongoc_version],
+    m4_ifset(
+        [mongoc_prerelease_version],
+        [mongoc_major_version.mongoc_minor_version.mongoc_micro_version-mongoc_prerelease_version],
+        [mongoc_major_version.mongoc_minor_version.mongoc_micro_version]))
 
 # bump up by 1 for every micro release with no API changes, otherwise
 # set to 0. after release, bump up by 1