allows for the usage of $(pkgconf --static --cflags libopenjp2) to produce
the proper CFLAGS for static linking. Relies on pkgconf rather than pkg-config
openjpeg provides libopenjp2.pc, so the require statements must refer to
libopenjp2 instead of openjp2.
Fixes#594
Signed-off-by: Stefan Weil <sw@weilnetz.de>