Make mkdtemp more portable

Taken from
https://github.com/gamma9mu/driftnet/commit/60659c3a7946c31e182655d6666977cdad3f1ea6
Use pkgconfig for linpng
Fix typo in button press_press-release
Allocate 64 instead of 34 bytes for path
Do not delete temporary files in adjunct mode as especified in help
Fix ifndef hidding needed values for non-gtk operation
Prevent backtrace due to unneccessary xfree on tmpdir
No makedepend
Refactor filename check code
Fix memstr() implementation non-working on 64bit systems. Closes: #677623

Taken from: https://bugzilla.redhat.com/show_bug.cgi?id=201412
Make driftnet read from pcap file. Closes: #647949
Link against libgif instead of ungif
Deal better with libpng errors
Update man file
Merge bug branches into upstream

 'bug/allocate-64-instead-34-for-path'
 'bug/do-not-delete-temp-files-in-adjunct-mode'
 'bug/fix-ifndef-hidding-needed-values'
 'bug/fix-typo-in-button_press-release'
 'bug/link-against-libgif-instead-of-ungif'
 'bug/make-drifnet-read-from-pcap-file'
 'bug/memstr-64-bit-spin'
 'bug/prevent-backtrace-due-to-unneccessary-xfree'
Merge branch 'bug/no-makedepend' into upstream

Conflicts:
	img.h
Merge branch 'bug/use-pkgconfig-for-libpng' into upstream

Conflicts:
	Makefile
Merge feature branches into upstream

 'feature/deal-better-with-libpng-errors'
 'feature/make-mkdtemp-more-portable'
 'feature/refactor-filename-check-code'
 'feature/update-man-file' into upstream