can you try out 
pecl download svn-beta
then replace the config.m4 with this file.
http://www.akbkhome.com/svn/pecl/svn/config.m4
run phpize;./configure;make clean;make install

make sure you have the development package / files installed for subversion.

# /opt/php/bin/phpize
Configuring for:
PHP Api Version:         20031224
Zend Module Api No:      20041030
Zend Extension Api No:   220040412
# ./configure
loading cache ./config.cache
checking host system type... i686-pc-linux-gnu
checking for gcc... gcc
checking whether the C compiler (gcc  ) works... yes
checking whether the C compiler (gcc  ) is a cross-compiler... no
checking whether we are using GNU C... yes
checking whether gcc accepts -g... yes
checking whether gcc and cc understand -c and -o together... yes
checking if compiler supports -R... no
checking if compiler supports -Wl,-rpath,... yes
checking for PHP prefix... /opt/php
checking for PHP includes... -I/opt/php/include/php -I/opt/php/include/php/main -I/opt/php/include/php/TSRM -I/opt/php/include/php/Zend
checking for PHP extension directory... /opt/php/lib/php/extensions/no-debug-zts-20041030
checking for re2c... exit 0;
checking for mawk... mawk
checking for svn support... yes, shared
checking for svn includes...
checking for svn libraries...
checking for svn_client_create_context in -lsvn_client-1... no
./configure: unset: `ac_cv_lib_svn_client-1_svn_client_create_context': not a valid identifier
configure: error: wrong svn lib version or lib not found

# ls /opt/subversion/include/
apr-0  neon  subversion-1

(These subdirectory contain header files)

Is there any configure-option to subversion that I may have missed?

try 
./configure --with-svn=/opt/subversion

# ./configure --with-svn=/opt/subversion

[...]

checking for svn support... yes, shared
checking for svn includes...  -I/opt/subversion/include/subversion-1 -I/opt/subversion/include/apr-0
checking for svn libraries... -lsvn_client-1 -lapr-0
checking for svn_client_create_context in -lsvn_client-1... no
./configure: unset: `ac_cv_lib_svn_client-1_svn_client_create_context': not a valid identifier
configure: error: wrong svn lib version or lib not found

does this show any output?
strings /opt/subversion/lib/libsvn_client-1.so | grep svn_client_create_context

Does this file exist?
/opt/subversion/lib/libsvn_client-1.a

It does:

# strings /opt/subversion/lib/libsvn_client-1.so | grep svn_client_create_context
svn_client_create_context

# file /opt/subversion/lib/libsvn_client-1.a
/opt/subversion/lib/libsvn_client-1.a: current ar archive

try this one.
http://www.akbkhome.com/svn/pecl/svn/config.m4

phpize;./configure=/opt/subversion

Still the same error:

checking for svn includes... checking for svn_client_create_context in -lsvn_client-1... no
./configure: unset: `ac_cv_lib_svn_client-1_svn_client_create_context': not a valid identifier
configure: error: wrong svn lib version or lib not found

(Also when removing config.cache before)

yet another go.. 
http://www.akbkhome.com/svn/pecl/svn/config.m4

This one should get you past the ./configure.. but may end up with problems at build time.

OK. Configure runs, make fails as you said:

# make
/bin/sh /root/svn-0.1/libtool --mode=compile gcc -I/opt/subversion/include/subversion-1 -I/opt/subversion/include/apr-0 -I. -I/root/svn-0.1 -DPHP_ATOM_INC -I/root/svn-0.1/include -I/root/svn-0.1/main -I/root/svn-0.1 -I/opt/php/include/php -I/opt/php/include/php/main -I/opt/php/include/php/TSRM -I/opt/php/include/php/Zend -I/opt/subversion/include/subversion-1 -I/opt/subversion/include/apr-0  -DHAVE_CONFIG_H  -g -O2   -c /root/svn-0.1/svn.c -o svn.lo
gcc -I/opt/subversion/include/subversion-1 -I/opt/subversion/include/apr-0 -I. -I/root/svn-0.1 -DPHP_ATOM_INC -I/root/svn-0.1/include -I/root/svn-0.1/main -I/root/svn-0.1 -I/opt/php/include/php -I/opt/php/include/php/main -I/opt/php/include/php/TSRM -I/opt/php/include/php/Zend -I/opt/subversion/include/subversion-1 -I/opt/subversion/include/apr-0 -DHAVE_CONFIG_H -g -O2 -c /root/svn-0.1/svn.c  -fPIC -DPIC -o svn.lo
/root/svn-0.1/svn.c: In function `init_svn_client':
/root/svn-0.1/svn.c:247: parse error before `*'
/root/svn-0.1/svn.c:253: `providers' undeclared (first use in this function)
/root/svn-0.1/svn.c:253: (Each undeclared identifier is reported only once
/root/svn-0.1/svn.c:253: for each function it appears in.)
/root/svn-0.1/svn.c: In function `php_svn_log_message_receiver':
/root/svn-0.1/svn.c:686: parse error before `const'
make: *** [svn.lo] Error 1

can you do 
gcc -v
and post the output.

Also can you send me a tarball of that directory in it's current state. (alan at akbkhome dot com)

beginning to run out of ideas..

I'm guessing that it cant find the definition of apr_array_header_t.. 

do you have ..../include/apr-0 installed in any other place or is it old headers.. ?

if you have another location, you could try --with-svn-apr=/location/of/apr ...

I ran make on your tarball and it compiled perfectly here. (with gcc-3.3.6)

No. Includes are in /opt/subversion/include/apr-0. Maybe it's my rather outdated gcc. I'll give it a try on another machine.

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