added quotes to AC_SUBST variables which might contain spaces.
In the AC_SUBST for CFLAGS_EXTRA we now pull in existing . Now with this, a user can pass in CFLAGS_EXTRA, get those put into CLFAGS, and NOT lose the configure handling of the CFLAGS variable.
This may help on some systems that are not cross compiling, BUT which need to have some unique CFLAGS data entered.
NOTE, I do not know how to add this to the help screen. That would be VERY nice to inform the users of. Also, we might want to add ways to do ASFLAGS, and LDFLAGS without messing with the REAL variables and having configure not auto probe them.