meson: simplify handling of pkgconfigdatadir=no, pkgconfiglibdir=no

meson: simplify handling of pkgconfigdatadir=no, pkgconfiglibdir=no

The idea was that those vars could be configured to 'no' to not install the .pc
files, or they could be set to '', and then they would be built but not
installed. This was inherited from the autoconf build system. This couldn't
work because '' is replaced by the default value. Also, having this level of
control doesn't seem necessary, since creating those files is very
quick. Skipping with 'no' was implemented only for systemd.pc and not the other
.pc files. Let's simplify things and skip installation if the target dir
is configured as 'no' for all .pc files.