Should I define _GNU_SOURCE?

[Victor Porton]

I am writing a software for Linux/Unix which could benefit from
execvpe() C function.

But this function is defined only when -D_GNU_SOURCE.

Question 1: Should I ALWAYS use -D_GNU_SOURCE for compiler options no
matter what are user options?

Question 2: Should I check existence of execvpe() by checking
#if _GNU_SOURCE
or with AC_CHECK_FUNC(execvpe)?

Question 3: Other advice?