Only use pkgconfig to detect imagemagick libs

As Debian remove Magick-config shell script (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=761927), i propose to only use pkgconfig to detect imagemagick libraries.

Original patch included in Debian package here : http://anonscm.debian.org/cgit/pkg-php/php-imagick.git/plain/debian/patches/imagemagick_pkgconfig_detection.patch