forward ccache to cmake via CMAKE_<LANG>_COMPILER_LAUNCHER

[kevinushey]

Problem

When R is configured with a compiler launcher baked into CXX (or CC) -- e.g. CXX = "ccache g++" -- building the bundled oneTBB fails during cmake configuration with:

CMake Error at cmake/compilers/GNU.cmake:65 (math):
  math cannot parse the expression: "/usr/bin/ccache: invalid option -- 'W' *
  1000 + /usr/bin/ccache: invalid option -- 'W'": syntax error, ...

Root cause

splitCompilerVar() in src/install.libs.R treats the first token of CXX as the compiler, so CXX = "ccache g++" was forwarded to cmake as CMAKE_CXX_COMPILER=ccache with g++ demoted into CMAKE_CXX_FLAGS. oneTBB's GNU.cmake then probes the assembler with ${CMAKE_CXX_COMPILER} ... -Wa,-v, i.e. ccache ... -Wa,-v. ccache rejects -W (invalid option -- 'W'), the version regex fails to match, and the leftover error text is fed to cmake's math(), which aborts the build.

Fix

CMake models compiler launchers via the dedicated CMAKE_<LANG>_COMPILER_LAUNCHER variable, not as part of the compiler command. This PR adds extractCompilerLauncher(), which pulls a leading ccache/sccache launcher (path- and .exe-tolerant) out of CC/CXX before splitting, and forwards it via -DCMAKE_C_COMPILER_LAUNCHER= / -DCMAKE_CXX_COMPILER_LAUNCHER=. The real compiler is left in place, so cmake's compiler/assembler probes run against it as intended, while ccache still wraps the actual compilation.

Tests

Adds cases to tests/test-install-libs.R covering launcher extraction (ccache, path-qualified ccache with trailing flags, sccache), and the no-op paths (plain compiler, unset variable).