#1 Support for g++-4.1

[Yann Dirson]

This patch attempts to modernize the code while keeping
changes minimal:
- fixed const issues
- activated compat mode for fltk
- "using namespace" here and there
- fixed a few includes

Be sure to "rm *.d" before running make, the shipped
file dependencies prevent the build to complete.

[Yann Dirson]