src/CMakeLists.txt: Use LUA52_LDFLAGS instead of LUA52_LIBRARIES

LUA52_LIBRARIES contains the -l flags. LUA352_LDFLAGS contains the same
information, plus the -L flags.

This is necessary on FreeBSD, where Lua is installed in /usr/local,
but /usr/local/lib isn't in the default search path of the linker.
LUA52_LDFLAGS specifies -L/usr/local/lib.