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.