[tz] Tzdb build issue

[Paul Eggert]

> CFLAGS="-Os -g -pipe" LDFLAGS="" CXXFLAGS="-Os -g -pipe"  \
>         CFLAGS='-DPORT_TO_C89  ' \
> 	make -j6 zic=zic cc=arm-linux-gnueabi-gcc ZDUMPDIR=/usr/sbin   ZFLAGS="-b slim"
> make[1]: Entering directory '/home/tzdb-2023c'

That merely tries to set CFLAGS in the environment of 'make', which is 
ineffective as one can see from the failing command, which was:

> arm-linux-gnueabi-gcc -DTZDIR='"/usr/share/zoneinfo"'    -c -o asctime.o asctime.c

with no "-DPORT_TO_C89" in sight.

I suggest that you instead give a CFLAGS=... argument to 'make', e.g., 
via this shell command:

   make -j6 zic=zic cc=arm-linux-gnueabi-gcc \
     ZDUMPDIR=/usr/sbin ZFLAGS="-b slim" \
     CFLAGS="-Os -g -pipe -DPORT_TO_C89"

There's no need to set CXXFLAGS as tzcode doesn't use C++.