fltk: bump to version 1.3.2

Fixes:
http://autobuild.buildroot.net/results/519/5195f7a8a38a5ba3ce5a0bf2a7649d390e461ee9/



The failure is related to bundled libpng conflicting with system libpng.
Which was a problem even before the libpng bump because the bundled
version for the previous fltk version has several security
vulnerabilities.
So disable bundled libraries and use global ones.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>