build: fix lua.c file generation

If Lua source path given to <lua_source> function is relative, the
output file is generated in the binary directory. At the same time if
the given path to be compiled to *.lua.c is absolute, the output
file is generated in source directory instead of the binary one. This
patch fixes the latter case, providing the valid behaviour for out of
source build type.

Needed for #4862

Reviewed-by: Sergey Kaplun <skaplun@tarantool.org>
Reviewed-by: Timur Safin <tsafin@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>