Using CMAKE to set the sysroot statically at the time we build
sac2c sets limits on the packaged version of sac2c. CMAKE sysroot
is versioned to match the XCode/SDK used to compile sac2c. This
versioned path **only** exists for a specific version of XCode. On other
setups, newer/older version of MacOS, and even newer/older version of
XCode, this path is probably invalid.

The solution is to use /usr/bin/cc instead, which is a wrapper around
clang that correctly sets the sysroot path information (and other
details besides).

Additionally this commit:
* move to cmake 3.19 base support (for better support on Apple M1)
* disable (manually) FAT binary generation (for Apple systems)
* use -mmacosx-version-min flag to correct set minimum support SDK
  version
* extend str.c; correct module/function comments; add STRstrip function
  to remove newline/space characters
* extend unit tests for strings (STRstrip)
* add platform/macos.c util functions (NEEDED?)
* add scripts/get-xcode-version.sh script (NEEDED?)