the bug may be fixed in "CVS", but where exactly ?
which files need amending ?

This should be fixed in CVS, try a snapshot from http://snaps.php.net
Alternatively, try this:
CC=gcc ./configure <normal configure options go here>

I have downloaded the newest one(php4-latest.tar.gz)
from cvs, but the bug still exits.
the following is the error message from make:

/bin/sh libtool --silent --mode=compile gcc  -Iext/ctype/ -I/tmp/php/ext/ctype/
In file included from /tmp/php/main/php.h:34,
                 from /tmp/php/ext/ctype/ctype.c:23:
/tmp/php/Zend/zend.h:59:19: unix.h: No such file or directory
/tmp/php/Zend/zend.h:71:25: mach-o/dyld.h: No such file or directory
In file included from /tmp/php/ext/ctype/ctype.c:23:
/tmp/php/main/php.h:88:18: unix.h: No such file or directory
make: *** [ext/ctype/ctype.lo] Error 1

the command before make is:
./configure --with-apxs=/usr/local/apache/bin/apxs \
            --with-mysql=/usr/local/mysql \
            CC=gcc

The confiog.cache file retains the incorrect information, 
so :- 
rm config.cache
CC=gcc ./configure <options>
make etc

On Solaris, I have also added 
export LD_RUN_PATH=/usr/lib:/usr/local/lib
This appears to work ... hope it helps

I believe that the source of this problem, at least on the solaris platform, is having /usr/ucb/cc in one's path before or instead of a real functioning cc. CC=gcc does the trick, or ln -s /usr/local/bin/gcc /usr/local/bin/cc would work too, as long as /usr/ucb/cc doesn't come first. The cc command is used to test for the presence of the files, hence this problem.