So setting your LD_LIBRARY_PATH, LD_RUN_PATH, and CC were all it took to fix your problem?
I'm actually having the exact same problem compiling php-4.2.1 on solaris8 as you described in your original post...
In file included from zend_compile.h:24,
from zend_language_parser.c:147:
zend.h:55:19: unix.h: No such file or directory
make[1]: [zend_language_parser.lo] Error 1
make[1]: Leaving directory `/opt/pkgdev/php-4.2.1/Zend'
make: *** [all-recursive] Error 1
Out of curiosity I mimicked your environment variables and made clean, configure, make, but my error persists. Did you perchance make any other changes?
-zak