Fix HardsubX builds

[PunitLodha]

Tracking issue for fixing HardsubX builds on all platforms. New feature was added to the rust crate to optionally enable compiling of HardsubX. These platforms need fixing to add that feature flag in the cargo build command and make any required platform specific changes. Also need to update the compilation docs wherever required.

I haven't checked all. Some might already be working correctly without need for any changes.

• [x] Linux/cmake #1443

• [x] Linux/build script #1443

• build_hardsubx and build

• [x] Linux/autoconf #1443

• Makefie.am and configure.ac

• [ ] Windows/(Release/Debug)-Full

• Issue with ffmpeg-sys-next build script. logs | Issue

• [ ] Mac/cmake (Need to confirm if Mac requires anything specific for cmake)

• [ ] Mac/build script

• build.command

• [x] Mac/autoconf

• Makefie.am and configure.ac

[shashwat1002]

@PunitLodha

for autoconf, the linux and mac scripts seem to be identical in master (I see the mac specific clause only adds new sources) So I am copying the autoconf script for mac for now. But I have no way of testing locally.

[PunitLodha]

Fixed by #1480 Thanks @prateekmedia