bug#26764: Problem building master

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

After doing 'guix pull' to get latest master branch, `guix environment
guix' fails while compiling the new sources:

Backtrace:
In ice-9/r4rs.scm:
  39: 19 [call-with-values #<procedure 6809cc0 at ice-9/eval.scm:416:20 ()> ...]
In ice-9/eval.scm:
 411: 18 [eval # #]
 411: 17 [eval # #]
In srfi/srfi-1.scm:
 573: 16 [map #<procedure 68128a0 at ice-9/eval.scm:416:20 (a)> (# # # # ...)]
In ice-9/eval.scm:
 387: 15 [eval # #]
 411: 14 [eval # #]
In ice-9/r4rs.scm:
  39: 13 [call-with-values #<procedure 6817cc0 at ice-9/eval.scm:416:20 ()> ...]
In ice-9/eval.scm:
 411: 12 [eval # #]
 411: 11 [eval # #]
In srfi/srfi-1.scm:
 575: 10 [map #<procedure 6865750 at ice-9/eval.scm:416:20 (a)> (# # # # ...)]
In ice-9/boot-9.scm:
 160: 9 [catch srfi-34 #<procedure 68653f0 at ice-9/eval.scm:416:20 ()> ...]
In ice-9/eval.scm:
 387: 8 [eval # #]
 481: 7 [lp (#<fluid 30> #<fluid 24> #<fluid 25>) (#f "x86_64-linux" #f)]
 481: 6 [lp (#<fluid 24> #<fluid 25>) ("x86_64-linux" #f)]
 481: 5 [lp (#<fluid 25>) (#f)]
In ice-9/r4rs.scm:
  39: 4 [call-with-values #<procedure 686b6c0 at ice-9/eval.scm:416:20 ()> ...]
In guix/gexp.scm:
 604: 3 [#<procedure 68f7a20 at guix/gexp.scm:604:2 (state)> #]
 626: 2 [#<procedure 4ea4370 at guix/gexp.scm:604:2 (builder)>
"/gnu/store/rzxs5lr5ay9n7s3657sf4y2hc4l9dnzk-bash-4.4.tar.xz-builder"]
 989: 1 [imported-modules ((guix build utils)) #:name ...]
In unknown file:
   ?: 0 [# # #<procedure 6e44d00 at guix/gexp.scm:991:29 (expr)> #]

ERROR: In procedure #<syntax-transformer mapm>:
ERROR: Wrong type to apply: #<syntax-transformer mapm>


I could reproduce the problem on two different machines.

Maxim