env.sh should not refer to global directories

[chqrlie]

The current version of env.sh has this contents:

echo 'setting C2 environment'
export C2_LIBDIR=~/c2_libs
export C2_PLUGINDIR=~/c2_plugins
export CC=clang
export CXX=clang++
#complete -W '-a -A -b -c -C -d -f -h -m -q -Q -r -s -S -t -T -v --check --create --fast --help --help-recipe --showlibs --showplugins --targets --test' c2c

Referring to subdirectories of the home directory is problematic:

• the version in development should not override plugins compiled for the installed version
• the c2 library used for development is now a submodule, C2_LIBDIR should refer to that.
  Furthermore CXX is no longer used, so the definition can be removed
  Also I am not sure CC should be forced as clang, which might not be available on the current system

I suggest this change:

echo 'setting C2 environment for development'
export C2_LIBDIR=$PWD/c2_libs
export C2_PLUGINDIR=$PWD/c2_plugins
export CC=clang
alias c2c="$PWD/output/c2c/c2c"
#complete -W '-a -A -b -c -C -d -f -h -m -q -Q -r -s -S -t -T -v --check --create --fast --help --help-recipe --showlibs --showplugins --targets --test' c2c

I would prefer this alternative approach:

• if c2c identifies that it is being run from an XXX/output/c2c/ subdirectory, it would use XXX/c2_libs and XXX/c2_plugins instead of the environment variables.
• remove the env.sh script
• simplify the install_plugins.sh script or better, make it a Makefile target.

This allows different checkouts to coexist and not interfere with an installed C2 compiler.