Fix building from MSYS shell

In debugger.pro, take path that checks and sets
QMAKE_CHK_EXISTS_GLUE if QMAKE_SH isn't empty.

Change-Id: I7c37acc5867c1ac3ab6da33db3eb2479c1d4213c
Reviewed-by: Alexey Pavlov <alexey.pawlow@gmail.com>
Reviewed-by: hjk <hjk121@nokiamail.com>