Use GCCVER only if defined
Created by: fxcoudert
Suggestion: right now, to use a specific version number, the user must edit the Makefile. With this change, GCCVER is used if it is defined. You can run:
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make atomes GCCVER=12, which will set the compilers togcc-12etc.
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make atomeswithout version specified will set the compilers togccetc.