Fix `make` on `srcdir != builddir`

[fd00]

$ cd /usr/src
$ git clone https://github.com/fragglet/lhasa.git
$ cd lhasa
$ autoreconf -fiv
$ cd /tmp/lhasa
$ /usr/src/lhasa/configure
$ make
/usr/src/lhasa/src/main.c:26:10: fatal error: lib/lha_arch.h: No such file or directory
   26 | #include "lib/lha_arch.h"
      |          ^~~~~~~~~~~~~~~~

[sezero]

It would be really nice if this gets merged

[Ghabry]

ack for merge. I'm currently submitting this to vcpkg and this requires working out-of-tree builds.

[fragglet]

Looks good, thanks!