* configure: Don't hardcode `grep -E`.

TeXLive still supports Solaris 5.10, where the system's `grep` doesn't
accept the `-E` option.  We thus introduce an `EGREP` variable that is set
to either `grep -E` or `-egrep`.

configure

@@ -15,10 +15,19 @@
 
 rm -f config.mk builds/unix/unix-def.mk builds/unix/unix-cc.mk
 
+if test -z "$EGREP"; then
+  if echo a | grep -E '(a|b)' >/dev/null 2>&1; then
+    EGREP="grep -E"
+  else
+    # if grep -E doesn't work, assume egrep does.
+    EGREP=egrep
+  fi
+fi
+
 # respect GNUMAKE environment variable for backward compatibility
 if test "x$GNUMAKE" = x; then
   if test "x$MAKE" = x; then
-    if test "x`make -v 2>/dev/null | grep -E 'GNU|makepp'`" = x; then
+    if test "x`make -v 2>/dev/null | $EGREP 'GNU|makepp'`" = x; then
       MAKE=gmake
     else
       MAKE=make
@@ -28,7 +37,7 @@ else
   MAKE=$GNUMAKE
 fi
 
-if test "x`$MAKE -v 2>/dev/null | grep -E 'GNU|makepp'`" = x; then
+if test "x`$MAKE -v 2>/dev/null | $EGREP 'GNU|makepp'`" = x; then
   echo "GNU make (>= 3.81) or makepp (>= 2.0) is required to build FreeType2." >&2
   echo "Please try" >&2
   echo >&2