Make the CMake build more pkg-config friendly

Build & install a libarchive.pc file, like autotools
Move .pc generation to a dedicated script (#55).
Merge branch 'master' into cmake-pkgconfig
Merge branch 'master' into cmake-pkgconfig
Merge remote-tracking branch 'upstream/master' into cmake-pkgconfig
Merge remote-tracking branch 'upstream/master' into cmake-pkgconfig
Merge remote-tracking branch 'upstream/master' into cmake-pkgconfig
Merge pull request #55 from NiLuJe/cmake-pkgconfig

Make the CMake build more pkg-config friendly