Not having pkg-config on Linux, LIBLO is not found

[JohannesLorenz]

This seems unnecessary, because zyn does find LIBLO without pkg-config, too.

Consequence: Building zyn without pkg-config on your Linux will fail:

src/Tests/PortChecker.cpp:11:10: fatal error: liblo-server.h: No such file or directory
   11 | #include <liblo-server.h>  // from rtosc's test directory
      |          ^~~~~~~~~~~~~~~~

This is because RTOSC_TEST_INCLUDE_DIR is actually being set in rtosc, but only if LIBLO was found:

CMakeLists.txt:    set(RTOSC_TEST_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/test CACHE INTERNAL "")
CMakeLists.txt:    set(RTOSC_TEST_LIB_DIR ${CMAKE_CURRENT_BINARY_DIR} CACHE INTERNAL "")

Fixing this might better wait or be done together with the windows-ci branch on Zyn.

[fundamental]

I'm fine if we solve this at the same time as the windows-ci branch changes