Restore includedir and libdir

Restore includedir and libdir. This was removed some time between 0.6.2 and 0.6.3 [1] causing pkg-config --libs and --ldflags to report empty -I and -L paths.

[1] v0.6.2...v0.6.3

See also:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=269990

CMakeLists.txt

@@ -38,6 +38,8 @@ SET(INCLUDE_INSTALL_DIR ${CMAKE_INSTALL_FULL_INCLUDEDIR} CACHE PATH "Installatio
 SET(PACKAGE ${PROJECT_NAME})
 SET(VERSION ${PROJECT_VERSION})
 SET(PACKAGE_STRING "${PACKAGE} ${VERSION}")
+SET(includedir ${INCLUDE_INSTALL_DIR})
+SET(libdir ${LIB_INSTALL_DIR})
 
 CONFIGURE_FILE(libdiscid.pc.in libdiscid.pc)
 CONFIGURE_FILE(versioninfo.rc.in versioninfo.rc)