Create optimized gcc build by default on Linux as well

[chros73]

Create optimized gcc build by default on Linux as well:

• make it configurable with OPTIMIZE_BUILD env var, it can be unset during build time by:
  • OPTIMIZE_BUILD=no ./build.sh ch
• use the following options:
  • MAKE_OPTS=-j4
  • CFLAGS="-march=native -pipe -O2 -fomit-frame-pointer"
• display + in title bar (instead of -) for optimized build
• don't allow packaging optimized build

Also update documentation (build instructions).