Squashed commit of branch 0.6.28

* installtools/target/*: Created path.inc to move some more or less
  hardcoded paths to a single source.  Not all stuff is there yet,
  update when needed.
* installtools/target/build: Use '#' as separator in the sed scripts
  and simplify the substitution of MAKEFILE_CFGDIR
* configure.py: Sanitize variables on the way out too and added '#'
  and '&' as unsafe characters.

[SerenityOS]
* configure.py, installtools/target/build: Support both gcc and clang.
* installtools/target/uninstall, installtools/target/postuninstall:
  Always remove both launcher and .ico in case $(serenity-launcher)
  ever changes.
* installtools/target/build, installtools/target/install: Install and
  use .ico if $(serenity-launcher) is false.  Note that `.ico`s are
  not yet supported for IconPath.
* doc/INSTALL.SerenityOS: Updated
* desktop/serenity.c: Remove unnecessary cast to void, and split lines
  between file and argv[0]