Improve version string generation for stable release source archives.

Add KiCadVersion.cmake to set the build version and branch name.

Update WriteVersionHeader.cmake to include KiCadVersion.cmake to set
the defaults in case no .git directory is found.

CMakeModules/WriteVersionHeader.cmake

@@ -28,7 +28,7 @@
 # version.h file is still created with KICAD_BUILD_VERSION set to
 # "no-vcs-found".
 if( NOT KICAD_BUILD_VERSION )
-    set( _wvh_version_str "no-vcs-found" )
+    include( ${CMAKE_MODULE_PATH}/KiCadVersion.cmake )
 
     # Attempt to detect if we have a git repo and set the version string.
     if( EXISTS "${SRC_PATH}/.git" )