src/Makefile: Introduce LUA_PKGNAME
src/Makefile: Introduce LUA_PKGNAME

The CFLAGS variable was being set by calling pkg-config with param
'lua5.3'. This made it impossible to build for lua 5.1 without editing
the Makefile.

Now the LUA_PKGNAME variable can be set on the make command line to
override the package name used for lua.

daurnimator/ldbus