Add more support to pkgconfig

[r2d2Proton]

Will PkgConfig be supported?
Is there any technical reason why it cannot or will not be supported?

I am looking at this now but any help is appreciated.

Thank you.

[dg0yt]

What kind of support do you ask for?
There is:

• vcpkg_find_acquire_program(PKGCONFIG) (but PKGCONFIG missing in documentation)
• vcpkg_fixup_pkgconfig(...)

[dg0yt]

vcpkg_find_acquire_program(PKGCONFIG) is also used by vcpkg_configure_make(...).

[dg0yt]

Pkgconfig support for manifest mode is just about to be created. Cf. #17481

[JackBoosY]

We should support automatic parsing of pkgconfig files and printing usage in vcpkg like cmake.