Use a function prototype for _ linkage probing

Merged Andreas Marek requested to merge master_pre_stage into master

The prototype chosen (char return type, no arguments) follows the usual autoconf approach for link tests. This is why compilers are likely to continue to support this construct even if there is a mismatch in types.

Future compilers are likely to reject implicit function declarations, and the checks will always fail with such compilers. (Implicit function declarations were removed from C in 1999.)

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