Fix build_desura.sh for ease of development.

Use 'set -e' rather than requiring '|| exit 1' after each command; the trouble
with that is that it's spotty, producing spurious success after failed builds.

Accept, as additional $1 commands, the names of the shell functions defined in
this script: clean, build_desura, build_cef. The current if/elif logic always
invokes 'clean' before any build activity; when a clean build includes
downloading and rebuilding Boost and all other dependent libraries, that
wastes considerable time. As a consequence, we no longer need recognize
"clean" explicitly in the elif chain. (But we must still recognize "pack"
because of its extra argument.)

Add a final else clause to fail if the verb is unrecognized, instead of doing
nothing and claiming success.

Turn on CMAKE_VERBOSE_MAKEFILE to help diagnose problems with compiler switches.

Use "$@" rather than plain $@ so arguments will be correctly forwarded rather
than reparsed at each level of indirection.

Use bash rather than plain Bourne sh so we can use 'declare' to test for shell
functions.
Add Linux support for BuildBoost.cmake.

Current logic assumes either vanilla Windows or MinGW. Add cases for Linux in
a couple places.
Change GNU.cmake switch from -std=c++0x to -std=c++11.

Desura source requires certain C++11 features (e.g. direct class-member
initialization) that require -std=c++11 rather than -std=c++0x.
Add Boost_INCLUDE_DIR to usercore/CMakeLists.txt.