pkgconfig: Define OPJ_STATIC for static linking with pkgconf

[Biswa96]

allows for the usage of $(pkgconf --static --cflags libopenjp2) to produce
the proper CFLAGS for static linking. Relies on pkgconf rather than pkg-config.

[Neumann-A]

This is also required for the CMake targets. Unfortunally openjpeg uses add_definitions instead of properly adding the definitions to the targets via target_compile_definitions.

I currently see

opj_jpip_test.c.obj : error LNK2019: unresolved external symbol __imp_get_index_from_JP2file referenced in function main
opj_jpip_test.c.obj : error LNK2019: unresolved external symbol __imp_destroy_index referenced in function main
opj_jpip_test.c.obj : error LNK2019: unresolved external symbol __imp_output_index referenced in function main

due to that

[Biswa96]

Feel free to add that. I was only seeing the static build issue in ffmpeg which uses pkgconfig files. And this change fixes it.