subversion17: add :apr dependency

Fixes #590.
gcc49: sandbox within series release number only

Refs #33948.
llvm35: specify branch for HEAD
combine conditionals
unconditionally build clang and libcxx
remove some explicit use of buildpath
simplify installing python bindings
inline variables
rm comment that probably isn't relevant anymore
combine conditionals
simplify caveats
rm comments that repeat the code
fixup! simplify installing python bindings
use block-form inreplace
relative paths
collect 10.6-specific code in one place
llvm33: add 3.3_1 bottle.
llvm34: add 3.4.2 bottle.
llvm35: add 3.5.0 bottle.