It's a bug in autoconf 2.13 (4.20 was created with 2.52, which causes a lot of problems on Linux).
THere are two things you can do:
1. rm configure && ./buildconf  (with autoconf 2.52 installed)
or
2. CC-gcc ./configure --your-switches

Derick

I appear to have autoconf 2.53
(from www.sunfreeware.com)

pkginfo -l SMCautoc | grep VERSION
VERSION: 2.53

Do I need to go back to 2.52?

2.53 should work too, but I can't promise anything :)

I still got the same error.  I will see if I can 2.52 somewhere.

{dcars1}[root]:/usr/src/php-4.2.1: rm configure && ./buildconf
rebuilding configure
rebuilding main/php_config.h.in
WARNING: Using auxiliary files such as `acconfig.h', `config.h.bot'
WARNING: and `config.h.top', to define templates for `config.h.in'
WARNING: is deprecated and discouraged.

WARNING: Using the third argument of `AC_DEFINE' and
WARNING: `AC_DEFINE_UNQUOTED' allows to define a template without
WARNING: `acconfig.h':

WARNING:   AC_DEFINE([NEED_MAIN], 1,
WARNING:             [Define if a function `main' is needed.])

WARNING: More sophisticated templates can also be produced, see the
WARNING: documentation.
autoheader: `main/php_config.h.in' is created
{dcars1}[root]:/usr/src/php-4.2.1: make
Making all in Zend
make[1]: Entering directory `/usr/share/src/php-4.2.1/Zend'
/bin/sh ../libtool --silent --mode=compile gcc -DHAVE_CONFIG_H -I. -I. -I../main   -D_POSIX_PTHREAD_SEMANTICS -I../TSRM  -g -O2 -prefer-non-pic -static -c -o zend_language_parser.lo `test -f zend_language_parser.c || echo './'`zend_language_parser.c
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 `/usr/share/src/php-4.2.1/Zend'
make: *** [all-recursive] Error 1

This seems a little unorthodox, but I have it working with autoconf 2.53,  I was able to reproduce this on several systems.

[step 1]
./configure --with-apache=../apache_1.3.24
--with-zlib-dir=/usr/local/lib --with-bz2=/usr/local/bin
--with-gd=/usr/local/lib 
--with-pgsql=/usr/local/pgsql --enable-track-vars --enable-ftp 

This will error out with the original error.

[step 2]
rm configure && ./buildconf  

This will error out with basically the same error.

[step 3]
export CC=gcc ./configure --with-apache=../apache_1.3.24
--with-zlib-dir=/usr/local/lib --with-bz2=/usr/local/bin
--with-gd=/usr/local/lib 
--with-pgsql=/usr/local/pgsql --enable-track-vars --enable-ftp 

Then it works OK.  I tried just doing the export CC=gcc
first, but that doesn't seem to help.  This seems a little weird to me, but I'm happy, it appears to be working now.

Thanks.

I have experienced exactly the same problem, but according to your comments and looking in the variables configured (within config_vars.mk file) for a correct installation of PHP 4.2.0, I concluded the following. Note that my environment is Solaris 8 (SunOS5.8), have installed gcc 3.0.3 and autoconf 2.53, and I want to install PHP 4.2.1.

- At configuration, prefix the "./configure <my-options>" command with C compiler (CC) and C pre-processor (CPP) flags as below,

      CC=gcc CPP="gcc -E" ./configure <my-options>

Congigure will set C compiler and pre-processor as above, and no more "can't open conftest.out" will appear. Then "make" will not produce any errors and installation of PHP 4.2.1 will be smooth and simple.

- I realised (looking in configure code) that file "conftest.out" is generated and manipulated by a program code in C called "conftest.c". Since compilation and run of the above code in done during configuration, there is an obvious connection between the code and the set C compiler.

- Since configure, for some reason, cannot locate the right compiler, I force gcc by prefixing the configure command with the required compiler flags (as suggested by mccaffityr).

I hope this helps to make a smooth installation of PHP 4.2.1.

Regards
Marsel

I had the same problem with compilation on Solaris8 intel.
Problem disappeared after installing libtool 1.4 from www.sunfreeware.com

Regards 
Fryderyk