I can't run mkfontdir at all on my computer with 2.7.10_beta2. See transcript below. It happens in any directory (with or without X fonts).
I tried to upgrade to _rc1 to see if it fixed it, but unfortunately that wouldn't run (see #97757).
==957==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60300000dcbf at pc 0x00010a1619d1 bp 0x7fff55acf950 sp 0x7fff55acf110
READ of size 1 at 0x60300000dcbf thread T0
*** This bug has been marked as a duplicate of bug 96905 ***
Sorry about opening the same bug twice. I had forgotten I had done so earlier this summer.
Bug numbers are cheap ;)