If you compile libpurple from scratch, you could use the following configure options:
user@host~# ./configure --enable-gnutls=yes --enable-nss=no --disable-gtkui --disable-consoleui --disable-screensaver --disable-sm --without-x --prefix=/usr
This will compile only libpurple. Otherwise, if you have already pidgin installed, libpurple is normally present in the /usr/lib or /usr/local/lib.
While within Phurple dir, invoke:
user@host~# phpize && ./configure --enable-phurple && make && make install
Since the extension is installed, you can simply add a line in your php.ini with which it'll be loaded by default. An other way would be to use the dl() php function at the top of your script
Note, that since the windows variant isn't exists yet, there are no installation instructions for this. Any help with windows builds is appreciated.