With rust-lang/rust#62281, rust compiles in mcmodel=medium mode
which is equivalent to GCC medany. This prevents linker relocation
errors code is placed outside the range `-0x80000000..0x7ffffffff`.

Some test cases check that a compiler flag is not present.  But
cc::Build loads additional flags from the CFLAGS and CXXFLAGS
environment variables.  If these are set, they might interfere with the
test cases.

Therefore we clear the CFLAGS and CXXFLAGS environment variables before
running a test that requires an absent flag.

* Detect and use `sccache` via `RUSTC_WRAPPER`

If no other C/C++ caching tool is found by inspecting `CC` and `CXX`,
`RUSTC_WRAPPER` is tested to see if an output-caching wrapper for
`rustc` is in use. If that is the case and it is a wrapper known to also
support C/C++ caching, use it.

(Also correct/clarify a misnamed variable that caused me some confusion
looking over the code.)

* Support RUSTC_WRAPPER on Windows and with absolute paths

When checking for possible `RUSTC_WRAPPER`s that we can use to cache
C/C++ output, allow for filename extensions (e.g. `sccache.exe`) and
absolute paths (e.g. `/usr/local/bin/sccache`).

Closes #473

…X builds (#481)

…ang++ on Windows (#483)

…488)

On windows, I have both MSVC 15 and 16 and found out that find_vs_version returns MSVC 16 while find_tool returns MSVC 15 linker.

* Fix android clang compiler path when building with Windows host

* Only use the cmd clang compiler if the exe clang doesn't exist