libass: allow compilation without a system font provider

If no system font provider is provided, then libass will fail to build
because it requires one by default. The error looks like this:

    configure: error: Either DirectWrite (on Windows), CoreText (on OSX),
    or Fontconfig(Linux, other) is required. If you really want to compile
    withouta system font provider, add --disable-require-system-font-provider

(typo is from the actual message)

But, as the error messages says, we can allow the compilation without a
system font provider if we use the above configure option, so let's do
it.

Fixes:
    http://autobuild.buildroot.net/results/3a4/3a4c07a0a54d5497fa8a4be2191856e286729637/
    http://autobuild.buildroot.org/results/d5a/d5a93b5a870de748259b097ec0b98b207cfaa303/


    and many others...

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
[yann.morin.1998@free.fr: move the --disable option in the !fontconfig
 case]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>