[FEATURE] Allow build with hardsubx using cmake

[thealphadollar]

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Fixes #964

To allow building with hardsubx, option has been added along with checking for the libraries it requires which have been cross-checked with Makefile.am

ccextractor/linux/Makefile.am

Line 330 in 3d6a9f4

ccextractor_CFLAGS += -DENABLE_HARDSUBX

Below is the build result after implementing the changes and using cmake.

make -j8 was successful in compiling the source code. Below is the command used on a file to test.

The required library for building with FFMPEG, "avfilter", was not listed in the required libraries and hence has been added to it. This was done in response to an error caused at the time of testing (due to absence of libavfilter-dev in my system), below is the error. Parameters used:
cd ./build
cmake -DWITH_FFMPEG=ON ../src/
make -j8

The tests were also done on a remote Digital Ocean droplet by forking the branch and hence are system independent and are working.