Make m4 and awk required via find_program

[mathomp4]

This is an attempt to do what @melven did in #259 but in a bit stricter sense.

We make both awk and m4 REQUIRED via find_program and rely on users to install in their base environment.

Will keep draft until @melven can test.

[melven]

Works mostly fine:

• shows errors when m4 or gawk are not available
• finds gawk and m4 if they are in the PATH on windows

Tests don't work due to escaping console commands on Windows for gawk:

The following works on Windows:

    COMMAND ${AWK} "\"/<start>/,/<stop>/\"" ${tmp} > ${tmp2}
    COMMAND ${AWK} "\"NR>2 {print last} {last=$$0}\"" ${tmp2} > ${actual}

but seems to make problems on Linux where the (current) variant should work:

    COMMAND ${AWK} '/<start>/,/<stop>/' ${tmp} > ${tmp2}
    COMMAND ${AWK} 'NR>2 {print last} {last=$$0}' ${tmp2} > ${actual}

[mathomp4]

@melven I just pushed a change that I hope makes it generic.

Can you re-pull this branch and try again?

[mathomp4]

Whoops. I was too clever. Hold please...

[mathomp4]

Okay. Now it doesn't crash on all our tests. @melven can you test the Windows side?

[melven]

Okay. Now it doesn't crash on all our tests. @melven can you test the Windows side?

Works fine now on Windows in my setup. Thanks!