--with-sfcgal requires path to config even when config is in system path

[robe]

When configuring debbie to build with sfcgal, I figured since I installed sfcgal in system (/usr/bin) installing using apt-get install from sid repo

./configure --with-sfgcal

would have been sufficient. Instead I got a compile error:

configure: error: sfcgal-config cannot be found. Please install sfcgal

I had to find where it got installed with:

dpkg -L libsfcgal-dev

and then pass in the full path

--with-sfcgal=/usr/bin/sfcgal-configu

Technically this is not a bug since our documentation says you have to pass in the full path, but would be nice for it to find it so its like other configure scripts.