Revert "Do not override environment CFLAGS and CXXFLAGS"

This reverts commit 80ebd60e7c. The
reason why we cleared CFLAGS/CXXFLAGS was because otherwise we get a
default value of -O2, which interferes with the defaults set in the
Makefile. (E.g. "make OPTIMIZE=0" should not pass -O2.)

configure.ac

@@ -55,6 +55,8 @@ if test "$sys_name" = sunos; then
 fi
 
 
+CFLAGS=
+CXXFLAGS=
 AC_PROG_CC
 AC_PROG_CXX