README: mention potential "Pre-C99 build failure"

[Jim Meyering]

A documentation change:

        * README: Describe potential "pre-C99 build failure", and work-around.

Index: README
===================================================================
RCS file: /fetish/cu/README,v
retrieving revision 1.27
diff -u -r1.27 README
--- README      17 Aug 2006 19:58:18 -0000      1.27
+++ README      19 Aug 2006 16:20:42 -0000
@@ -61,6 +61,22 @@
 
 
 ***********************
+Pre-C99 build failure
+-----------------------
+
+There is a new, implicit build requirement:
+To build the coreutils from source, you should have a C99-conforming
+compiler, due to the use of declarations after non-declaration statements
+in several files in src/.  There is code in configure to find and, if
+possible, enable an appropriate compiler.  However, if configure doesn't
+find a C99 compiler, it continues nonetheless, and your build will fail.
+If that happens, simply apply the included patch using the following
+command, and then run make again:
+
+  cd src && patch < c99-to-c89.diff
+
+
+***********************
 HPUX 11.x build failure
 -----------------------