b2: recipe doesn't respect tools.build:compiler_executables

[TheClonerx]

b2 has an option b2/*:use_cxx_env=True which is intended to pass a custom compiler and flags when using the b2/*:toolset=cxx option.
However, the current recipe does not respect the following conf values:

• tools.build:compiler_executables
• tools.build:cxxflags
• tools.build:defines
• tools.build:exelinkflags

Because of this, compilation of b2 will use the wrong compiler and flags.

Additionally, because this recipe will only respect buildenv.CXX & buildenv.CXXFLAGS, even when set correctly, no link options are passed to the build.
On my system that has multiple gcc installations this causes symbol errors at startup due to trying to use shared linking with libstdc++11, despite me setting conf.tools.build:exelinkflags to ["-static-libgcc", "-static-libstdc++"]