"--fast-math" caused build failure with clang for
homebrew/science/delly-0.7.2

Apparently, clang only likes this option when it's referred to as
"-ffast-math"

Closes Homebrew/homebrew#50507.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

`/usr/bin/<tool>` will be a popup stub under such configuration.

The idea is to let `scm/git` to handle all of git location resolution
throughout Homebrew codebase.

We cannot really recover from this, but at least we'll provide a clearer
error message than trying to call `exec` with an empty string.

Some things can break in hard to debug ways if users or build scripts
set these variables, causing our otherwise quite robust Ruby wrappers
to fail. In theory, we could also use `--disable-rubyopt`, but this is
not supported in Ruby 1.8 (and we still care about it).

Use the same interpreter for the shims that is also used to run the main
Homebrew process (the one invoked via `brew`). The magic basically lies
in executing `ruby` with the `-x` option (supported since at least 1.8)
and in the following shebang line.

These weren't being filtered correctly if the compiler was being called
as g++ instead of gcc.

Fixes mistydemeo/tigerbrew#371.

Fixes mistydemeo/tigerbrew#371.
Fixes Homebrew/homebrew#47949.

`echo` is missing since f3f6552c.

It's needed by `cgo` which is used by a lot of Go software.