add shell test to example makefiles

[stevelr]

when run under windows powershell, the example makefiles fail with multi-line commands that use \ for line continuation, but the error given isn't helpful for the user to diagnose. We should build a check into the makefile to test the shell capabilities and display an informative error.
Something like the following (pseudocode) should be added to build/makefiles/*.mk:

if $SHELL != "bash" && $SHELL != "zsh" && $SHELL != "fsh" ; then
    if ! $(multi \
        line \
        command ); then
        echo "this makefile expect a bash-compatible shell. Please start `bash`, `zsh`, or a similar shell before running make commands"
        exit
     fi
fi

the first if only runs the test if the shell isn't a common shell type.

[stale]

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