Library man pages don't mention "-D_GNU_SOURCE" (newlib)

[Keith Thompson <Keith dot S dot Thompson at gmail dot com>]

The exp10 function is a GNU extension, as the man page acknowledges.

The exp10(3) man page (available if you install the "cygwin-doc"
package) doesn't mention that you need to compile with "-D_GNU_SOURCE"
to use it. It seems to be inordinately difficult to find this
information.

The corresponding GNU man page, for example exp10(3) on Ubuntu, says:

SYNOPSIS
       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <math.h>

I suggest adding similar wording to the Cygwin/newlib man pages for
functions that require it.

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