thirdparty: lcms2: append flags found by pkg-config if available

thirdparty: lcms2: append flags found by pkg-config if available

This change allows to get all required CFLAGS/LDFLAGS in case of static only
build.

Fixes a buildroot build failure (see [1], [2] and [3]).

[1] http://autobuild.buildroot.net/results/5ce/5cee20afd8bef5268832cddcb3a5270746be7a57
[2] http://lists.busybox.net/pipermail/buildroot/2016-November/177187.html
[3] http://lists.busybox.net/pipermail/buildroot/2016-November/177188.html

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
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Depends on https://github.com/uclouvain/openjpeg/pull/866