CMake: Use find_program instead of which to find gcc toolchain

[cvanbeek13]

This updates the CMake configuration to use the CMake find_program() function to locate the toolchain gcc application. The existing implementation uses which, which is only available on Linux. find_program() works cross-platform. By setting HINTS to $ENV{PATH}, CMake looks for the program using the PATH environment variable, in the same way which would on Linux.

[dgarske]

Hi Corry,

It was great talking with you earlier and thank you for the fix. I am tagging @gojimmypi to pull in this fix. To accept code contributions we need a signed contributor agreement. If you'd like to get setup with that agreement just send me an email.

Thanks,
David Garske, wolfSSL

[gojimmypi]

Hi @cvanbeek13 -

I have some cmake improvements, including alternate toolchain locations - in #598 (very much WIP ATM, suggestions welcome).

If you'd like me to test and put up a PR so you don't need to fuss with the contributor agreement, I'm happy to do that.

Otherwise, it's always nice to have new contributors.

Cheers