The BOOST_STACKTRACE_HAS_BACKTRACE check doesn't seem to work with clang

When configuring cmake on ubuntu 24.04 with clang, the check for libbacktrace doesn't seem to work, even when libbacktrace is installed in the default location.

For reference, gcc puts libbacktrace here:
`/usr/lib/gcc/x86_64-linux-gnu/13/include/backtrace.h`

Configuring with clang, we see:
```
[build] -- Performing Test BOOST_STACKTRACE_HAS_BACKTRACE
[build] -- Performing Test BOOST_STACKTRACE_HAS_BACKTRACE - Failed
```

This can be corrected by explicitly telling the library where the header resides (the extra escapes are to pass the literal quotes all the way in to the header file as a define):
```
-DCMAKE_CXX_FLAGS="-DBOOST_STACKTRACE_BACKTRACE_INCLUDE_FILE=\\\"/usr/lib/gcc/x86_64-linux-gnu/13/include/backtrace.h\\\""
```

Can the cmake check of BOOST_STACKTRACE_HAS_BACKTRACE be modified to look at the gcc install location?

Start at the CMake logic for the BOOST_STACKTRACE_HAS_BACKTRACE check and review how it locates backtrace.h when configuring with clang. Reproduce the failed check on Ubuntu 24.04 with clang and the GCC libbacktrace installation, then confirm configuration succeeds without explicitly defining BOOST_STACKTRACE_BACKTRACE_INCLUDE_FILE.

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