There are several common reasons for this:
You have an old version of GTK installed somewhere. You should remove this old copy, but note that this may break applications that have been compiled against the old version.
pkg-config (or another component of GTK) isn't in your path, or there is an old version on your system. Type:
pkg-config gtk+-2.0 --modversion
to check for both of these. If it returns a value different from what you expect, then you have an old version of GTK on your system.
The ./configure script can't find the GTK libraries. As ./configure compiles various test programs, it needs to be able to find the GTK libraries. See the question above for help on this.
If none of the above help, then have a look in config.log, which is generated by ./configure as it runs. At the bottom will be the last action it took before failing. If it is a section of source code, copy the source code to a file and compile it with the line just above it in config.log. If the compilation is successful, try executing it.