[openh264] only build shared or static library

[mcmtroffaes]

Describe the pull request

• What does your PR fix?

  Currently, when installing openh264, both the static and shared library get built and potentially installed, even though only a static or shared one is requested by vcpkg. For example, on x64-linux:

  # before patch
  $ ./vcpkg install openh264
  $ ls installed/x64-linux/lib/
  libopenh264.a  libopenh264.so  libopenh264.so.2.1.0  libopenh264.so.6  pkgconfig
  # after patch
  $ ./vcpkg install openh264
  $ ls installed/x64-linux/lib/
  libopenh264.a  pkgconfig

  This PR incorporates an upstream patch from meson: Use pkg-config generator and some cleanups cisco/openh264#3351 to ensure that openh264 respects meson's default_library option, resolving this issue (since this is the mechanism also used by vcpkg to request either the static or shared library).

• Which triplets are supported/not supported? Have you updated the CI baseline?

  No changes.

• Does your PR follow the maintainer guide?

  Yes.

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes.

[mcmtroffaes]

My CI has fully cycled now and I can now confirm that this PR also fixes the following error on osx:

https://travis-ci.com/github/mcmtroffaes/ffmpeg-msvc-build/jobs/501838063#L371

Reason is that the FindFFMPEG script picks up the dynamic library instead of the static library:

https://travis-ci.com/github/mcmtroffaes/ffmpeg-msvc-build/jobs/501838063#L317

[NancyLi1013]

Thanks for your fixing @mcmtroffaes.

[mcmtroffaes]

Thanks for merging!