Don't update po files on normal build, add target --update-po for this task.
Fix wrong handling of CCFlags and CXXFlags which caused the build to hang if CFLAGS were not already set.
Don't make checks for header files and functions mandatory.
Update some method names to latest API changes in Waf.
Various other small fixes.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@2661 ea778897-0a13-0410-b9d1-a72fbfd435f5