build: make error if sh.exe in PATH

[FreddieAkeroyd]

If sh.exe is in the windows PATH then make will fail at some point. This usually happens if the Git "bin" rather than "cmd" directory is added to PATH. Fix if possible.

[FreddieAkeroyd]

This seems to happen in Makefiles using "echo", so quite a bit builds, usually fails makeing runIOC.bat

[FreddieAkeroyd]

Setting SHELL and MAKESHELL from the external environment does not help. The only workaround was to set SHELL = cmd inside a Makefile itself

[FreddieAkeroyd]

To test:

Open CMD window in an iocBoot directory
Set "PATH=c:\program files\git\bin;%PATH%"
make clean uninstall && make

check it fails

merge patch and repeat