tools/bison: hardcode path to m4 with STAGING_DIR_HOST

tools/bison: hardcode path to m4 with STAGING_DIR_HOST

Force bison to ignore the M4 environment variable and hardcode it to the
locally built m4 during build operations using the relocatable path
variable STAGING_DIR_HOST.

This allows bison to continue to function while we are forcefully avoiding
autoreconf and other autoconf and automake-like operations by giving a
fake path to m4 with the M4 environment variable.

The specific path can still be overridden independently from the
environment within the line of invocation that runs bison by setting
STAGING_DIR_HOST within the command, so document this in the help printout.

Tested-by: Georgi Valkov <gvalkov@gmail.com> # MacOS
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>