* Add GNUInstallDirs for standard installation directories
Distributions are given standard variables for already existing hooks.
Multiarch libdirs is taken care of automagically.
Raises minimum cmake version by a little.
* Handle CMAKE_INSTALL_xxx being absolute paths for .pc file generation
In some cases the CMAKE_INSTAL_{BIN,MAN,DOC,LIB,INCLUDE}DIR variables
may turn out to be absolute paths in which case prepending ${prefix} in
the pkg-config .pc files will result in incorrect values.
For .pc file generation, figure out if these variables are absolute and
omit the prefix in the configured file when so.
See: ab25e4b7ed
c7bccf0515