gnu: liba52: Build shared library.

* gnu/packages/patches/liba52-enable-pic.patch,
  gnu/packages/patches/liba52-link-with-libm.patch,
  gnu/packages/patches/liba52-set-soname.patch,
  gnu/packages/patches/liba52-use-mtune-not-mcpu.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/video.scm (liba52)[source]: Add patches.
  [native-inputs]: Add autoconf, automake, and libtool.
  [arguments]: Add "--enable-shared" to configure-flags.
  Add 'bootstrap' phase.