checking whether cairo's Microsoft Windows font backend could be enabled... no
(requires a Win32 platform)
checking for cairo's PNG backend... 
./configure: line 24790: --exists: command not found
./configure: line 24790: --exists: command not found
./configure: line 24790: --exists: command not found
configure: WARNING: Could not find libpng in the pkg-config search path
checking whether cairo's PNG backend could be enabled... no
configure: error: requested PNG backend could not be enabled

But:
$ pkg-config --exists libpng12
$ echo $?
0
$ 

The problem persists even with
$ export PKG_CONFIG=`which pkg-config`
$ echo $PKG_CONFIG                                                             
                                             
/usr/bin/pkg-config
$ 

This is all on Slackware 10.2,
$ gcc -v                                                                       
                                             
Reading specs from /usr/lib/gcc-lib/i486-slackware-linux/3.3.6/specs
Configured with: ../gcc-3.3.6/configure --prefix=/usr --enable-shared
--enable-threads=posix --enable-__cxa_atexit --disable-checking --with-gnu-ld
--verbose --target=i486-slackware-linux --host=i486-slackware-linux
Thread model: posix
gcc version 3.3.6

The libpng came with slack:
/var/log/packages/libpng-1.2.14-i486-1_slack10.2


Cheers,
Andrej