…uild (closes #3049)

#3046 skipped liquid-dsp on Windows MSVC because cl.exe rejects C99
`_Complex` arithmetic that liquid-dsp's source code uses throughout
(`liquid.h:473` error C2061 was just the first surface; the .c sources
also use complex `+` / `*` operators that MSVC's C compiler doesn't
support).  This unblocks the Windows build path so future digital-mode
consumers can use liquid-dsp on all three platforms.

Approach: a separate ExternalProject_Add sub-build invokes CMake with
`-DCMAKE_C_COMPILER=clang-cl.exe`.  clang-cl is LLVM's MSVC-ABI-
compatible frontend (ships with VS 2022's "C++ Clang Compiler for
Windows" component) and supports `_Complex` natively while producing
MSVC-compatible `.lib` output.  The resulting `liquid.lib` links into
the main AetherSDR/MSVC binary without ABI drama.

Linux + macOS path unchanged — still `add_subdirectory` in-tree.
Link line now uses `if(TARGET liquid-static)` instead of
`$<$<NOT:$<CXX_COMPILER_ID:MSVC>>:liquid-static>` so a Windows build
without clang-cl (rare; only an unusually slim VS install) cleanly
omits liquid-static rather than failing the link.

Verified:
- Linux x86_64 in-tree build still clean (this commit)
- Windows clang-cl path needs CI to confirm (cannot test locally)

If `clang-cl.exe` isn't on PATH the configure logs an informational
"liquid-dsp: clang-cl not found" message and continues; the only
consumer-visible effect is that `liquid-static` is absent.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Windows CI on the previous commit failed at the ExternalProject configure
step:

  CMake Error at C:/Program Files/CMake/share/cmake-3.31/Modules/Platform/
  Windows-Clang.cmake:168 (message):
    supported.  Use either clang/clang++ or clang-cl as all C, C++, CUDA,
    ...

liquid-dsp's project() declares `LANGUAGES C CXX`, and CMake's
Windows-Clang platform module refuses to mix clang-cl for C with MSVC
cl.exe for C++ — both must use a clang variant.  Setting only
CMAKE_C_COMPILER leaves the inner project's C++ language detection on
default cl.exe, which trips the consistency check.

Fix: also pass `-DCMAKE_CXX_COMPILER=${CLANG_CL_EXE}`.  The C++ compiler
isn't actually invoked (BUILD_EXAMPLES / AUTOTESTS / BENCHMARKS / SANDBOX
all disabled, and liquid-dsp's library sources are pure C) but the
declared language is enough to trigger CMake's platform-module check.

Linux build still clean (this commit).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>