Apache failed to compile in environments in which the `which` command
was not present (such as Docker).
This is fixed upstream (by using `command -v` instead) in
https://github.com/apache/httpd/commit/05297d7b00f0cb1f20d3ff14f4409097df2615fe
but not yet released.

This commit patches the configure script in a similar way, so that
the `which` command is not used.

See merge request nexedi/slapos!1736