Add --cc= syntax for selecting compilers
Unify compiler selection logic

This unifies the logic for selecting a compiler between superenv and
stdenv. A variation of superenv's `determine_cc`, which now returns a
symbol, has been moved into the shared ENV extension. Stdenv uses the
result of this directly (as it's always used symbols), while superenv
translates that back into a compiler string.

This also has the effect of disabling HOMEBREW_USE_(gcc|llvm|clang) in
stdenv, which have already been marked as deprecated for some time, and
enables the HOMEBREW_CC= environment variable syntax from superenv in
stdenv.
Document --cc= in the manpage
thrift: fix --HEAD

Closes #22218.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
vcsh 1.20130829

Closes #22217.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
boost-bcp 1.54.0

Closes #22208.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
xctool 0.1.10

Closes #22206.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
groonga 3.0.7

Closes #22186.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
arangodb: use bundled v8
ArangoDB 1.3.3

Closes #22174.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
xcproj 0.1

Closes #22193.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
UPX: --HEAD change official repository https://www.pysol.org:4443/hg/upx.hg/

upx 3.10

Closes #22099.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
play: change HEAD repo

Added check to not apply relative symlink hack to play file for 2.2.x(HEAD and devel)

Closes #22111.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>