cmake: Add PKG_CONFIG_PATH option
cmake: Add PKG_CONFIG_PATH option

Add PKG_CONFIG_PATH option to be able to specify install directory
for pkgconfig file. This change also fixes a typo (`share/pkgconfg`
-> `share/pkgconfig`).

google/libprotobuf-mutator,vitalybuka/libprotobuf-mutator,google/libprotobuf-mutator,vitalybuka/libprotobuf-mutator