Bug fix, installation improvements, and GCC/macOS version updates

Bug fix

Issue #511: the shape of coindexed multidimensional array component now yields the correct answer. 💠

Installer updates

1. If install.sh builds GCC on macOS Catalina, it finds and uses the requisite macOS software development kit (SDK) header files. ⛰️
2. GCC 10.1.0 is now the default minimum version used or installed by install.sh. Users may force the installer to use a lower version by invoking install.sh with the -f, -c, and -C arguments. 🆕
3. Pull request (PR) #705 fixes an error in the compiler version-check code invoked by install.sh.
4. PR #705 switches the GCC download mechanism to git to address two issues: 📫
   • GCC development branches: the previous default (svn) can no longer be used because GCC has adopted git.
   • GCC released versions: the previous default (ftp) is disabled on many systems for security reasons.
5. Other minor installation improvements. 🏗️