Expect GTK 2.10 libs to link against by default and remove GTK210 flag. If anyone wants to build on Windows against GTK 2.6 or 2.8 use 'make GTK280=1'.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@2883 ea778897-0a13-0410-b9d1-a72fbfd435f5