pstoedit: add X11 dependency

Otherwise fails with `ld: library not found for -lXext`:
```
/bin/sh ../libtool --tag=CXX   --mode=link clang++  -g -O2 -DINTERNALBOOL -no-undefined -module  -o libp2edrvmagick++.la -rpath /usr/local/Cellar/pstoedit/3.62_1/lib/pstoedit drvmagick++.lo initlibrary.lo -L/usr/local/Cellar/imagemagick/6.8.7-7/lib -lMagick++-6.Q16 -lMagickWand-6.Q16 -lMagickCore-6.Q16  libpstoedit.la -lgd 
libtool: link: clang++  -o .libs/libp2edrvmagick++.0.so -bundle  .libs/drvmagick++.o .libs/initlibrary.o   -L/usr/local/Cellar/imagemagick/6.8.7-7/lib /usr/local/Cellar/imagemagick/6.8.7-7/lib/libMagick++-6.Q16.dylib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib -L/usr/local/Cellar/freetype/2.5.2/lib -L/opt/X11/lib /usr/local/Cellar/imagemagick/6.8.7-7/lib/libMagickWand-6.Q16.dylib /usr/local/Cellar/imagemagick/6.8.7-7/lib/libMagickCore-6.Q16.dylib -llcms2 -lfftw3 -lfontconfig -lfreetype -lXext -lSM -lICE -lX11 -lXt -lbz2 -lz -lltdl ./.libs/libpstoedit.dylib -lstdc++ -ldl -lm -lgd  -O2   -pthread
ld: library not found for -lXext
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [libp2edrvmagick++.la] Error 1
make: *** [install-recursive] Error 1
```