fix: propagate M4 env variable to flex subprocess

Hi all,

I was trying to build Postgres with Meson specifying the binaries of the
tools such as bison, flex and m4. I found that the m4 binary wasn't being
used by flex, and digging into this led me to the pgflex wrapper.

The pgflex wrapper runs flex with an explicit environment, so it doesn't
inherit environment variables from the parent process. However, flex can
use the M4 env variable and/or the PATH (via execvp) to find the m4 macro
processor.

Thus, since flex honors the M4 env variable, it should be propagated to the
subprocess environment if it's set in the parent environment. This patch
fixes it.

Let me know what you think!

Regards,

Javier