I have two similar debian systems. One works fine the other has problems. From what I have been able to work out, on the system with problems, it looks like the code for checking for dlopen is not finding the function in the library. The beta3 code seems to find it but the rc1 code has a problem.

# grep dlopen php-4.0B3/config.cache
ac_cv_lib_c_dlopen=${ac_cv_lib_c_dlopen=no}
ac_cv_lib_dl_dlopen=${ac_cv_lib_dl_dlopen=yes}
# grep dlopen php-4.0RC1/config.cache
ac_cv_lib_c_dlopen=${ac_cv_lib_c_dlopen=no}
ac_cv_lib_dl_dlopen=${ac_cv_lib_dl_dlopen=no}

When we go to do the make it has problems at:
make[3]: Entering directory `/usr/local/src/php/php-4.0RC1/ext/standard'
/bin/sh ../../libtool --silent --mode=compile gcc -DHAVE_CONFIG_H -I. -I/usr/local/src/php/php-4.0RC1/ext/standard -I..
/.. -I../../Zend -I/usr/local/src/php/php-4.0RC1 -I/usr/include/apache-1.3 -I/usr/informix/incl/esql -I/usr/local/src/p
hp/php-4.0RC1/ext/xml/expat/xmltok -I/usr/local/src/php/php-4.0RC1/ext/xml/expat/xmlparse  -DEAPI=1 -DXML_BYTE_ORDER=12
 -D_REENTRANT -g -O2 -I/usr/informix/incl/esql -Wall  -c array.c
/bin/sh ../../libtool --silent --mode=compile gcc -DHAVE_CONFIG_H -I. -I/usr/local/src/php/php-4.0RC1/ext/standard -I..
/.. -I../../Zend -I/usr/local/src/php/php-4.0RC1 -I/usr/include/apache-1.3 -I/usr/informix/incl/esql -I/usr/local/src/p
hp/php-4.0RC1/ext/xml/expat/xmltok -I/usr/local/src/php/php-4.0RC1/ext/xml/expat/xmlparse  -DEAPI=1 -DXML_BYTE_ORDER=12
 -D_REENTRANT -g -O2 -I/usr/informix/incl/esql -Wall  -c base64.c
/bin/sh ../../libtool --silent --mode=compile gcc -DHAVE_CONFIG_H -I. -I/usr/local/src/php/php-4.0RC1/ext/standard -I..
/.. -I../../Zend -I/usr/local/src/php/php-4.0RC1 -I/usr/include/apache-1.3 -I/usr/informix/incl/esql -I/usr/local/src/p
hp/php-4.0RC1/ext/xml/expat/xmltok -I/usr/local/src/php/php-4.0RC1/ext/xml/expat/xmlparse  -DEAPI=1 -DXML_BYTE_ORDER=12
 -D_REENTRANT -g -O2 -I/usr/informix/incl/esql -Wall  -c basic_functions.c
basic_functions.c:324: `php_if_dl' undeclared here (not in a function)
basic_functions.c:324: initializer element is not constant
basic_functions.c:324: (near initialization for `basic_functions[201].handler')
basic_functions.c: In function `php_info_basic':
basic_functions.c:708: warning: implicit declaration of function `php_info_dl'
make[3]: *** [basic_functions.lo] Error 1
make[3]: Leaving directory `/usr/local/src/php/php-4.0RC1/ext/standard'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/usr/local/src/php/php-4.0RC1/ext/standard'
make[1]: *** [all] Error 1
make[1]: Leaving directory `/usr/local/src/php/php-4.0RC1/ext'
make: *** [all-recursive] Error 1   

I have places all the config.* files, the configure command line (configure.cmd) and the output from configure (configure.out) and make (make.out) into an archive at:
http://www.wave.co.nz/~ashepherd/php-4RC1.files.tgz