cmake: unbreak JXL build on FreeBSD

[jbeich]

Regressed by #6. Affects FreeBSD, other BSDs and maybe even Nix due to using different prefix for each package. To reproduce install libjxl outside of /usr and /usr/local thus not part of default C compiler search path.

$ pkg-config --libs libjxl
-L/libjxl_prefix/local/lib -ljxl

$ export PKG_CONFIG_PATH=/libjxl_prefix/lib/pkgconfig
$ cmake -G Ninja -B _build
$ cmake --build _build
[...]
ld: error: unable to find library -ljxl
ld: error: unable to find library -ljxl_cms
ld: error: unable to find library -ljxl_threads
c++: error: linker command failed with exit code 1 (use -v to see invocation)