Allow BUILD_REVISION to be defined at compilation time to manually specify the SVN revision, overriding the number derived from keywords.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@9010 c06c8d41-db1a-0410-9941-cceddc491573

Created by  ennewalker  on February 8, 2009
LCXMLLOYRE6UWNTREC62EVW3DQHXX7DCY7ZFCDPT4FZBOEVZBYEQC
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