the pkgconfig module automatically specified to install the
pkgconfig file to {libdir}/pkgconfig. Default settings in meson
already include multiarch-directories like x86_64-gnu-linux into
the libdir. pkgconfig usually does not check inside multiarch-dirs
for any pkgconfig-files.
to make this a bit more flexible, this commit introduces the
install_dir attribute for pkgconfig.generate. if it is set, the
default install path will be overridden by the users input