Add pkg-config export to CMake script

[OPNA2608]

Based on & includes #222.

Working on closing downstream issue https://gitlab.com/ubports/development/apps/lomiri-camera-app/-/issues/209.

The QMake script has code for generating a pkg-config file, which the CMake one lacks.

qzxing/src/QZXing-components.pri

Lines 464 to 468 in 6ea2b31

	# pkg-config support
	CONFIG += create_pc create_prl no_install_prl
	QMAKE_PKGCONFIG_DESTDIR = pkgconfig
	QMAKE_PKGCONFIG_LIBDIR = ${prefix}/lib
	QMAKE_PKGCONFIG_INCDIR = ${prefix}/include

This adds code to the CMake script to also generate a pkg-config file, which allows the unconditional use of pkg-config to gather flags for an installed QZXing library.

An example:

↪ cat /nix/store/i6gd51yg61b0ipfcan10q30phpkzz4bc-qzxing-3.3.0/lib/pkgconfig/QZXing.pc 
prefix=/nix/store/i6gd51yg61b0ipfcan10q30phpkzz4bc-qzxing-3.3.0
exec_prefix=${prefix}
libdir=/nix/store/i6gd51yg61b0ipfcan10q30phpkzz4bc-qzxing-3.3.0/lib
includedir=/nix/store/i6gd51yg61b0ipfcan10q30phpkzz4bc-qzxing-3.3.0/include/qzxing

Name: Qzxing
Description: Qzxing Library
Version: 3.3.0
Cflags: -DQZXING_MULTIMEDIA -DQZXING_QML -DDISABLE_LIBRARY_FEATURES -I${includedir}
Libs: -L${libdir} -lqzxing