build our own copy of autotools

currently only used by the "float" GAP package
normaliz: needs boost
remove unused ncurses build
Use clang / clang++ as compiler, and enable libc++ for the latter

This fixes issues where projects using c++0x or c++11 could not use
the gmpxx C++ interface -- in particular, this affected 4ti2.
Add 4ti2 and glpk packages
gap: switch from .tar.gz (261M) to .tar.bz2 (218M)
gap: remove more occurrences of hardcoded prefix
4ti2: fix install names of three more binaries
unify common.mk and Makefile a bit more
clean build-tools; define the build-tools path in a central location
glpk: fix install.sh
fix_install_names.sh: binary paths must be absolute or relative paths

Previously, we pre-pended $PREFIX, which was convenient in some
situations but made it difficult to use the script in a more general
context. The new behavior is more in line with virtually every other
tool out there.
make it possible to omit compile.sh / install.sh in some cases

Specifically, if they confirm to some standard pattern.
run-script.sh: add $TOOLSDIR/bin to PATH
float: simplify build.sh now that autotools are in PATH
gap: fix broken build
simplify / unify GAP package build rules
add simple realpath implementation
make use of realpath in GAP and Singular and also the .app script