The reformat_dat_file.pl script, added by 372728b0, supported
all the necessary options to make it work in a VPATH build, but the
makefile invocations didn't take VPATH into account. Fix that.

Discussion: https://postgr.es/m/20181115185303.d2z7wonx23mdfvd3@alap3.anarazel.de
Backpatch: 11-, where 372728b0 was merged