clang 12 and gcc 10 updates on cloud9

[Gary Ferland]

The thread titled "build error HEAD & clang" shows that the clang and gcc now offered on Macs has gone well beyond the compilers we have on cloud9, and generate warnings with our HEAD.  I asked Brian to upgrade both and he did, just before the holidays.

Here is Brian's email on the new compilers:

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gcc 10.3.0 built and installed in /usr/local .
clang+llvm 12.0.1 built against gcc 10.0.3 and installed in /usr/local/llvm12 .
You'll need /usr/local/lib64 and /usr/local/lib at the head of your LD_LIBRARY_PATH .
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NB - this setup is different from our previous versions of these compilers.  It might take some hand tuning of paths get things right.

thanks,
Gary