Update local-gcc.md

The file docs/local-gcc.md predates both the upcxx wrapper compiler and our configure script and therefore is written for the user constructing their own link like (from upcxx-meta, for instance) and the idea that the CXX environment variable was the main (only?) knob relevant to measures to be deployed at library build time.

With the upcxx wrapper as our primary recommended use case, this part of our documentation is now out-of-date. The focus of should be updated to stress getting a correct installation of upcxx suited to both direct compilation and use via our CMake package, in addition to supporting correct use of upcxx-meta.

At a minimum we need to include --with-ldflags=-Wl,... as a recommendation.

Additionally, the organization of the document should stress the use of configure options (over CXX) if necessary.

It may also be appropriate to drop the current item number 2, which suggests use of a 2-line shell script as the C++ compiler (since --with-ldflags=-Wl,... is nearly equivalent and more robust).

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