Advise CFLAGS= operand, not in environment, when configuring

Backport from master.

etc/DEBUG

@@ -16,7 +16,8 @@ make the debugging easier.  Here's the configure-time options we
 recommend (they are in addition to any other options you might need,
 such as --prefix):
 
-  CFLAGS='-O0 -g3' ./configure --enable-checking='yes,glyphs' --enable-check-lisp-object-type
+  ./configure --enable-checking='yes,glyphs' --enable-check-lisp-object-type \
+    CFLAGS='-O0 -g3'
 
 The CFLAGS value is important: debugging optimized code can be very
 hard.  (If the problem only happens with optimized code, you may need

etc/PROBLEMS

@@ -126,7 +126,7 @@ This version of GCC is buggy: see
 You can work around this error in gcc-4.5 by omitting sibling call
 optimization.  To do this, configure Emacs with
 
- CFLAGS="-g -O2 -fno-optimize-sibling-calls" ./configure
+ ./configure CFLAGS="-g -O2 -fno-optimize-sibling-calls"
 
 ** Emacs compiled with GCC 4.6.1 crashes on MS-Windows when C-g is pressed
 

nt/INSTALL

@@ -52,7 +52,8 @@ build will run on Windows 9X and newer systems).
      You can pass other options to the configure script.  Here's a
      typical example (for an in-place debug build):
 
-       CFLAGS='-O0 -g3' ./configure --prefix=/d/usr/emacs --enable-checking='yes,glyphs'
+       ./configure --prefix=/d/usr/emacs --enable-checking='yes,glyphs' \
+         CFLAGS='-O0 -g3'
 
   3. After the configure script finishes, it should display the
      resulting configuration.  After that, type
@@ -461,7 +462,8 @@ build will run on Windows 9X and newer systems).
   A few frequently used options are needed when you want to produce an
   unoptimized binary with runtime checks enabled:
 
-     CFLAGS='-O0 -g3' ./configure --prefix=PREFIX --enable-checking='yes,glyphs'
+     ./configure --prefix=PREFIX --enable-checking='yes,glyphs' \
+       CFLAGS='-O0 -g3'
 
   Once invoked, the configure script will run for some time, and, if
   successful, will eventually produce a summary of the configuration