gollvm: update README to include requirement for 'm4'

A working copy of 'm4' is needed to build gollvm. Call this out in
the installation instructions.

Updates golang/go#38728.

Change-Id: I67df113d447af75114239c35341fdbd4c92bdf27
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/240559
Reviewed-by: Cherry Zhang <cherryyz@google.com>

README.md

diff --git a/README.md b/README.md
index ccb448a..65e7369 100644
--- a/README.md
+++ b/README.md
@@ -66,7 +66,7 @@
 
 ## Building gollvm with cmake and ninja <a name="cmakeninja"></a>
 
-You'll need to have an up-to-date copy of cmake on your system (3.6 or later vintage) to build Gollvm, as well as a C/C++ compiler (V5.0 or later for Clang, or V6.0 or later of GCC).
+You'll need to have an up-to-date copy of cmake on your system (3.6 or later vintage) to build Gollvm, as well as a C/C++ compiler (V5.0 or later for Clang, or V6.0 or later of GCC), and a working copy of 'm4'.
 
 Create a build directory (separate from the source tree) and run 'cmake' within the build area to set up for the build. Assuming that 'workarea' is the directory created as above: