Fedora 20 strlen()

Fedora 20. I used autobuild/synfigstudio-linux-build.sh

My build failed with ‘strlen not declared in this scope’ in trgt_png_spritesheet.cpp.

I added the line "#include " to synfig/synfig-core/src/synfig/string.h and the build worked as expected. I’m not sure if this is a problem in the build tree or Fedora.

FWIW, I built synfig on a Fedora 19 machine a few weeks ago with no errors.

Please open a pull request for this patch. If it fails it would be easy to fix.