Update GCC default-minimum version to 10.1.0 & fix compiler version check

[rouson]

coverage on master

Summary of changes

Set install.sh to default to building GCC 10.1.0 if an earlier version is detected and no other version is specified. This can still be over-rided for users who want to build OpenCoarrays with an older version by passing the -f, -c, and -C arguments (or their longer equivalents).

Rationale for changes

1. This ensures co_broadcast handles derived type arguments.
2. This also updates the gcc download to use git because GCC has moved from svn to git and
   because many users cannot access the GCC ftp server for security reasons.
3. This merge also fixes problems with prerequisites/acceptable_compiler.f90, which failed when the acceptable compiler version matched the actual version.

Additional info and certifications

This pull request (PR) is a:

• Bug fix (replaces GCC download URL's that don't work for the latest releases)
• Feature addition (provides derived-type co_broadcast by default)
• Other, Please describe:

I certify that

• I certify that:
  • I have reviewed and followed the contributing guidelines
  • I will wait at least 24 hours before self-approving the PR to give another
    OpenCoarrays developer a chance to review my proposed code
  • I have not introduced errant white space (no trailing white space or white space errors may
    be introduced)
  • I have added an explanation of what these changes do and why they should be included
  • I have checked to ensure there aren't other open Pull Requests for the same change
  • I have you written new tests for these changes
  • I have successfully tested these changes locally
  • I have commented any non-trivial, non-obvious code changes
  • The commits are logically atomic, self consistent and coherent
  • The commit messages follow best practices
  • Test coverage is maintained or increased after this is merged

Code coverage data

[codecov]

Codecov Report

Merging #705 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #705   +/-   ##
=======================================
  Coverage   54.53%   54.53%           
=======================================
  Files           3        3           
  Lines        2923     2923           
=======================================
  Hits         1594     1594           
  Misses       1329     1329