add encoder cabac support
Fix openh264.pc file installation

If there's no pkgconfig subdirectory yet in our
install prefix, create it first, otherwise we'll
end up with a file called 'pkgconfig' instead.
Add .pc file generation to PHONY targets as version change in Makefile needs to cause a rebuild.
Merge pull request #1459 from tp-m/pkgconfig-install-fix

Fix openh264.pc file installation
Merge pull request #1460 from zaheerm/pkgconfig

Add .pc file generation to PHONY targets
Merge pull request #1457 from ruil2/enc_cabac

add encoder cabac support