diff --git a/racket/src/configure b/racket/src/configure
index 6d95e527d4..63bd11a103 100755
--- a/racket/src/configure
+++ b/racket/src/configure
@@ -3268,7 +3268,16 @@ if test "${enable_ios}" != "" ; then
   SUB_CONFIGURE_EXTRAS="${SUB_CONFIGURE_EXTRAS} LDFLAGS="'"'"${LDFLAGS}"'"'
 fi
 
+
 if test "${enable_oskit}" = "no" ; then
+  # Auto-set CC, CPP, etc.
+
+  if test "${CFLAGS}${CC}${CPP}" = "" ; then
+    starts_out_default=yes
+  else
+    starts_out_default=no
+  fi
+
   ac_ext=c
 ac_cpp='$CPP $CPPFLAGS'
 ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
@@ -4195,6 +4204,18 @@ ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
 ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
+
+  if test "$starts_out_default" = "yes" ; then
+    if test "$GCC" = "yes" ; then
+      # Auto selection of C compiler picked GCC, so we
+      # assume it also picked the GNU preprocessor; move
+      # auto-picked CFLAGS to PREFLAGS so that compilation
+      # and preprocessing are in sync (e.g., for optimization flags)
+      PREFLAGS="${CFLAGS} ${PREFLAGS}"
+      CPPFLAGS="${CFLAGS} ${CPPFLAGS}"
+      CFLAGS=""
+    fi
+  fi
   if test "$AS" = '' ; then
     AS="${ac_tool_prefix}as"
     as_was_set=no
diff --git a/racket/src/racket/configure.ac b/racket/src/racket/configure.ac
index 48766f53d7..f7f19853a9 100644
--- a/racket/src/racket/configure.ac
+++ b/racket/src/racket/configure.ac
@@ -539,10 +539,31 @@ if test "${enable_ios}" != "" ; then
   SUB_CONFIGURE_EXTRAS="${SUB_CONFIGURE_EXTRAS} CPPFLAGS="'"'"${PREFLAGS}"'"'
   SUB_CONFIGURE_EXTRAS="${SUB_CONFIGURE_EXTRAS} LDFLAGS="'"'"${LDFLAGS}"'"'
 fi
-    
+
+
 if test "${enable_oskit}" = "no" ; then
+  # Auto-set CC, CPP, etc.
+  
+  if test "${CFLAGS}${CC}${CPP}" = "" ; then
+    starts_out_default=yes
+  else
+    starts_out_default=no
+  fi
+  
   AC_PROG_CC
   AC_PROG_CPP
+
+  if test "$starts_out_default" = "yes" ; then
+    if test "$GCC" = "yes" ; then
+      # Auto selection of C compiler picked GCC, so we
+      # assume it also picked the GNU preprocessor; move
+      # auto-picked CFLAGS to PREFLAGS so that compilation
+      # and preprocessing are in sync (e.g., for optimization flags)
+      PREFLAGS="${CFLAGS} ${PREFLAGS}"
+      CPPFLAGS="${CFLAGS} ${CPPFLAGS}"
+      CFLAGS=""
+    fi
+  fi
   if test "$AS" = '' ; then 
     AS="${ac_tool_prefix}as"
     as_was_set=no