Replace custom checks with Autoconf macros
Removing environ in #19142 (moved) didn't work on BSDs but the check in configure.ac
can be simplified. Autoconf provides the AC_CHECK_DECL
macro for these kinds of checks and turns the current check into a oneliner.
The malloc(0)
check can also be simplified by replacing it with the AC_FUNC_MALLOC
macro.
These are only two candidates I've found so far, there are probably more hence this ticket.