Handle missing git info when building

externals/cmake-modules/GetGitRevisionDescription.cmake.in

@@ -33,6 +33,10 @@ else()
 endif()
 
 if(NOT HEAD_HASH)
-	file(READ "@GIT_DATA@/head-ref" HEAD_HASH LIMIT 1024)
-	string(STRIP "${HEAD_HASH}" HEAD_HASH)
+	if(EXISTS "@GIT_DATA@/head-ref")
+		file(READ "@GIT_DATA@/head-ref" HEAD_HASH LIMIT 1024)
+		string(STRIP "${HEAD_HASH}" HEAD_HASH)
+	else()
+		set(HEAD_HASH "Unknown")
+	endif()
 endif()