Fix -Werror=cast-function-type with GCC 8

[rbuj]

export CFLAGS="-Werror=cast-function-type -Wno-deprecated-declarations"
./autogen.sh --prefix=/usr
make
sudo make install

[sc0w]

travis fails in fedora:

Traceback (most recent call last):
  File "/usr/bin/itstool", line 1611, in <module>
    doc.merge_translations(translations, opts.lang, strict=opts.strict)
  File "/usr/bin/itstool", line 999, in merge_translations
    lcnode.setProp(attr, origlang)
  File "/usr/lib64/python3.7/site-packages/libxml2.py", line 3588, in setProp
    if ret is None:raise treeError('xmlSetProp() failed')
libxml2.treeError: xmlSetProp() failed
make[3]: *** [Makefile:602: pt/pt.stamp] Error 1
make[3]: Leaving directory '/rootdir/gsearchtool/help'
make[2]: *** [Makefile:443: all-recursive] Error 1
make[2]: Leaving directory '/rootdir/gsearchtool'
make[1]: *** [Makefile:511: all-recursive] Error 1
make[1]: Leaving directory '/rootdir'
make: *** [Makefile:443: all] Error 2
!!! ERROR: run command [docker exec -t mate-utils-fedora-build /rootdir/src_build].
The command "./docker-build --name ${DISTRO} --verbose --config .travis.yml --build autotools" exited with 1.

[raveit65]

travis fails in fedora:

Known issue
#210
itstool/itstool#36
This can happen randomly with every language.
And it is possible that this error will be gone if you start build again.

[raveit65]

Now, the build passed :)

[lukefromdc]

A build with my normal build options (not messing with -Werror) builds everything fine.
mate-search-tool runs as before
mate-disk-usage-analyzer runs as before
mate-system-log runs as before
mate-dictionary runs same as before both from the panel applet and from the command line. Note that if from the command line you look up something NOT in the database (e.g. the string "test2") and then afterwards run a valid lookup (I used the string "test") you get
(mate-dictionary:9199): Gdict-CRITICAL **: 17:03:45.914: _gdict_show_error_dialog: assertion 'GTK_IS_WIDGET (widget)' failed

both with this PR and with master, so for this PR it can be disregarded.

[lukefromdc]

Build with
export CFLAGS="-Werror=cast-function-type -Wno-deprecated-declarations"
also completed fine