auto/make: Fix various issues with building OTEL

There were at least a couple of issues with building OTEL support.

It only worked with GNU make due to the use of ifeq, even gmake had some
issues.

Debug builds were broken due to trying to pass --debug to cargo which is
the default and isn't a valid option.

This 'fixes' things by doing 'release' builds of OTEL by default.

Passing D=1 to make will generate 'debug' builds but this as previously
with D= etc, only works with GNU make.

We make use of the '--emit link=' rustc option to place the libotel.a
static library into build/lib

This is good, it consolidates the static libraries into one place and it
simplifies the build scripts.

While we're at it pretty print the cargo command by default.

Fixes: 9d3dcb800 ("otel: add build tooling to include otel code")
Link: <https://github.com/nginx/unit/pull/1520#issuecomment-2556265063>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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