Fix one warning on non-MSVC compilers.

[ArthurSonzogni]

Some Microsoft's specific #pragma were left in the code. They causes the
following errors to be printed:

/tmp/antlr4/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp:1351:9:
warning: unknown pragma ignored [-Wunknown-pragmas]
may be unsafe. Consider using _dupenv_s instead.
        ^
/tmp/antlr4/runtime/Cpp/runtime/src/atn/ParserATNSimulator.cpp:1361:9:
warning: unknown pragma ignored [-Wunknown-pragmas]

This patch removes the #pragma macros and uses the matching compiler
definition only on MSVC compiler.

[ArthurSonzogni]

Note: The "contributors.txt" will be updated first in: #2899. So I won't do it here.

[mike-lischke]

Not all users build the runtime using cmake. So your change will leave them then with new warnings.

[ArthurSonzogni]

Not all users build the runtime using cmake. So your change will leave them then with new warnings.

Indeed! Thanks!
I haven't thought about this requirement.