package/doxygen: bump version to 1.8.13, switch to cmake

package/doxygen: bump version to 1.8.13, switch to cmake

Doxygen switched to cmake with release 1.8.10:
http://www.stack.nl/~dimitri/doxygen/manual/changelog.html#log_1_8_10

and added support for flex 2.6.0 with release 1.8.11:
http://www.stack.nl/~dimitri/doxygen/manual/changelog.html#log_1_8_11

Fixes
http://autobuild.buildroot.net/results/ba1/ba1444709523c59393aaaa3710e9933bafacc6b5/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>