"make DESTDIR=/path/to/dir/ install" fails with:
Making install in po
make: Entering directory `/tmp/claws-mail/po'
make: /path/to/dir/usr/share: Command not found
make: *** [install-data-yes] Error 127
make: Leaving directory `/tmp/claws-mail/po'
make: *** [install-recursive] Error 1
That seems to be a problem of $(mkdir_p) being undefined, which in turn is a problem in aclocal.m4 (AM_GNU_GETTEXT > AM_PO_SUBDIRS > AM_PROG_MKDIR_P).