fatal error: opening dependency file deps/fpending.d: No such file or directory
This has been appearing for a while...
This is the beginning part of deps/*.d file, so it is not file specific, but some mechanism to create *.d file. (mkdep ?)

h97# which mkdep
/usr/bin/mkdep
h97# grep mkdep /bulklog/emacs26-26.0.50nb20170420/build.log 
h97# grep depend /bulklog/emacs26-26.0.50nb20170420/build.log
fpending.c:60:1: fatal error: opening dependency file deps/fpending.d: 
	No such file or directory

at least following line at /etc/mk.conf may work

GIT_REVISION.master= ce88155d83

this came from following line

| * |   ce88155d83 [2017-02-02 21:35:51 -0500] ; Merge: fixes and updates to scroll margin (Bug#5718) @Noam Postavsky

Looking at git log, my suspect is 2f8935099c

* | 2f8935099c [2017-03-17 11:33:48 -0700] Fixups for GNU Make switchover @Paul Eggert
* | 65faa7bcb5 [2017-03-17 11:33:47 -0700] Switch from Automake to GNU Make @Paul Eggert
* | 9c07e888a7 [2017-03-17 03:22:23 -0700] Don't suggest Mailutils on MS-Windows @Paul Eggert
* | 3c46ac7d65 [2017-03-17 10:16:44 +0100] Fix bug: Range-check integer alpha frame parm value @Thien-Thi Nguyen
* | 6b2f3ba02a [2017-03-17 10:25:46 +0100] Fix Bug#26127 @Michael Albinus

GIT_REVISION.master= 2f8935099c

will work
Major change at 65faa7bcb5