Fix phpdbg.1 man page installation when build != src directory

Some PHP compilation methods (e.g. Redhat PHP rpms) make SAPI's build a clean directory. The addition of the phpdbg.1 man page breaks this method, as it's assuming the build directory is the source directory.

This affects a build I'm creating for PHP 5.6.0RC2
https://github.com/webtatic-rpms/php56w/blob/master/php56.spec#L1344

This PR fixes the problem by forcing the Makefile to get the man page from the source directory.

an example I think recreates the problem:

    cd /usr/src/php-src/sapi
    git clone https://github.com/krakjoe/phpdbg
    cd ../
    ./buildconf --force
    mkdir ./build-phpdbg
    
    pushd ./build-phpdbg
    ln -s ../configure
    ./configure --enable-phpdbg
    popd
    
    make -C build-phpdbg -j8
    make -C build-phpdbg install-phpdbg