Add --skip-if-not-found to meson install

[julianneswinoga]

Sometimes when a project is partially built we only want to install a subset of the possible targets. Example:

• main project:
  • foo_app -> depends on libfoo. tagged as 'foo_app'
  • bar_app -> depends on libbar. tagged as 'bar_app'
• subprojects
  • libfoo
  • libbar

When building foo_app (meson compile foo_app) libfoo is built because it is a dependency. When installing foo_app to test it (meson install --no-rebuild --tags foo_app,runtime) meson will try to install all targets that are tagged as runtime, regardless if they were built or not. This gives the error:

ERROR: File 'libbar.so' could not be found

because libbar is automatically tagged with 'runtime'. Since it is a subproject dependency, we can't add a specific install_tag so we need to rely on the automatic 'runtime' tag.

This is a toy example, but the repo I am working with has many executable targets with varying library dependencies, so compiling + installing only a subset is needed for a nice developer experience.