Library man pages don't mention "-D_GNU_SOURCE" (newlib)
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
The corresponding GNU man page, for example exp10(3) on Ubuntu, says:
#define _GNU_SOURCE /* See feature_test_macros(7) */
I suggest adding similar wording to the Cygwin/newlib man pages for
functions that require it.