build-sys: add --with-pkgconfigdir

build-sys: add --with-pkgconfigdir

It seems we need a way how to override the default pkg-config install directory.

default:
	$ ./configure
	$ grep 'pkgconfigdir ='  Makefile
	pkgconfigdir = ${usrlib_execdir}/pkgconfig

user-defined:
	$ ./configure --with-pkgconfigdir=/usr/share/pkgconfig
	$ grep 'pkgconfigdir ='  Makefile
	$ pkgconfigdir = /usr/share/pkgconfig

Addresses: https://github.com/karelzak/util-linux/issues/793
Signed-off-by: Karel Zak <kzak@redhat.com>