Allow CFLAGS from configure's environment to override automatic CFLAGS.

Previously, configure would add any switches that it chose of its own
accord to the end of the user-specified CFLAGS string.  Since most
compilers process these left-to-right, this meant that configure's choices
would override the user-specified flags in case of conflicts.  We'd rather
that worked the other way around, so adjust the logic to put the user's
string at the end not the beginning.

There does not seem to be a need for a similar behavior change for CPPFLAGS
or LDFLAGS: in those, the earlier switches tend to win (think -I or -L
behavior) so putting the user's string at the front is fine.

Backpatch to 9.4 but not earlier.  I'm not planning to run buildfarm member
guar on older branches, and it seems a bit risky to change this behavior
in long-stable branches.

configure

@@ -4386,6 +4386,10 @@ else
   fi
 fi
 
+# CFLAGS we determined above will be added back at the end
+user_CFLAGS=$CFLAGS
+CFLAGS=""
+
 # set CFLAGS_VECTOR from the environment, if available
 if test "$ac_env_CFLAGS_VECTOR_set" = set; then
   CFLAGS_VECTOR=$ac_env_CFLAGS_VECTOR_value
@@ -4397,7 +4401,7 @@ fi
 # but has its own.  Also check other compiler-specific flags here.
 
 if test "$GCC" = yes -a "$ICC" = no; then
-  CFLAGS="$CFLAGS -Wall -Wmissing-prototypes -Wpointer-arith"
+  CFLAGS="-Wall -Wmissing-prototypes -Wpointer-arith"
   # These work in some but not all gcc versions
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -Wdeclaration-after-statement" >&5
 $as_echo_n "checking whether $CC supports -Wdeclaration-after-statement... " >&6; }
@@ -4904,7 +4908,12 @@ if test "$PORTNAME" = "win32"; then
   CPPFLAGS="$CPPFLAGS -I$srcdir/src/include/port/win32 -DEXEC_BACKEND"
 fi
 
-# Check if the compiler still works with the template settings
+# Now that we're done automatically adding stuff to CFLAGS, put back the
+# user-specified flags (if any) at the end.  This lets users override
+# the automatic additions.
+CFLAGS="$CFLAGS $user_CFLAGS"
+
+# Check if the compiler still works with the final flag settings
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether the C compiler still works" >&5
 $as_echo_n "checking whether the C compiler still works... " >&6; }
 cat confdefs.h - <<_ACEOF >conftest.$ac_ext

configure.in

@@ -414,6 +414,10 @@ else
   fi
 fi
 
+# CFLAGS we determined above will be added back at the end
+user_CFLAGS=$CFLAGS
+CFLAGS=""
+
 # set CFLAGS_VECTOR from the environment, if available
 if test "$ac_env_CFLAGS_VECTOR_set" = set; then
   CFLAGS_VECTOR=$ac_env_CFLAGS_VECTOR_value
@@ -425,7 +429,7 @@ fi
 # but has its own.  Also check other compiler-specific flags here.
 
 if test "$GCC" = yes -a "$ICC" = no; then
-  CFLAGS="$CFLAGS -Wall -Wmissing-prototypes -Wpointer-arith"
+  CFLAGS="-Wall -Wmissing-prototypes -Wpointer-arith"
   # These work in some but not all gcc versions
   PGAC_PROG_CC_CFLAGS_OPT([-Wdeclaration-after-statement])
   PGAC_PROG_CC_CFLAGS_OPT([-Wendif-labels])
@@ -488,7 +492,12 @@ if test "$PORTNAME" = "win32"; then
   CPPFLAGS="$CPPFLAGS -I$srcdir/src/include/port/win32 -DEXEC_BACKEND"
 fi
 
-# Check if the compiler still works with the template settings
+# Now that we're done automatically adding stuff to CFLAGS, put back the
+# user-specified flags (if any) at the end.  This lets users override
+# the automatic additions.
+CFLAGS="$CFLAGS $user_CFLAGS"
+
+# Check if the compiler still works with the final flag settings
 AC_MSG_CHECKING([whether the C compiler still works])
 AC_TRY_LINK([], [return 0;],
   [AC_MSG_RESULT(yes)],