configure: remove predefined -O2 optimization flag

CFLAGS_OPT variable forces to use -O2 optimization flag denying the
possibility to use different optimization flag values.

Remove -O2 flag from CFLAGS_OPT.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
---
diff -urpN prboom-2.5.0.orig/configure.ac prboom-2.5.0/configure.ac
--- prboom-2.5.0.orig/configure.ac	2019-06-20 14:07:45.058481879 +0200
+++ prboom-2.5.0/configure.ac	2019-06-20 14:09:05.066238825 +0200
@@ -48,7 +48,7 @@ else
 fi
 
 dnl --- cph: work out flags to pass to compiler
-CFLAGS_OPT="-O2 -fomit-frame-pointer"
+CFLAGS_OPT="-fomit-frame-pointer"
 AC_C_COMPILE_FLAGS(-Wall)
 dnl --- Option to enable debugging
 AC_ARG_ENABLE(debug,AC_HELP_STRING([--enable-debug],[turns on various debugging features, like range checking and internal heap diagnostics]),,enable_debug="no")