--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -44,7 +44,9 @@
 message(STATUS "FreeDV version: ${FREEDV_VERSION}")
 
 # Work around for not using a svn working copy.
-add_definitions(-DSVN_REVISION="Unversioned directory")
+set(SVN_REVISION "Unversioned directory" CACHE STRING
+    "Allow build environment to set revision.")
+add_definitions(-DSVN_REVISION=${SVN_REVISION})
 add_definitions(-D_NO_AUTOTOOLS_)
 
 
