re enable Fortran optimization flag on windows

partial revert of 299cdcdc29
from #696, was not explained why that was needed

Makefile.system

@@ -986,12 +986,8 @@ COMMON_OPT = -O2
 endif
 
 ifndef FCOMMON_OPT
-ifeq ($(OSNAME), WINNT)
-FCOMMON_OPT = -O0 
-else
 FCOMMON_OPT = -O2 -frecursive
 endif
-endif