Fix bootstrap-cmake-linux.sh to respect CC/CXX environment variables

The script was hardcoding CXX=clang++ CC=clang, ignoring any
user-provided environment variables. Now uses shell parameter
expansion to default to clang only if CC/CXX are not already set.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/bootstrap-cmake-linux.sh

@@ -5,10 +5,14 @@ builddir=${curdir}/build
 rm -rf ${builddir}
 mkdir ${builddir}
 
-# with lld linker
-#  -DCMAKE_TOOLCHAIN_FILE=cmake/lld-linux.toolchain.cmake 
+# Use environment CC/CXX if set, otherwise default to clang
+: ${CC:=clang}
+: ${CXX:=clang++}
 
-cd ${builddir} && CXX=clang++ CC=clang cmake \
+# with lld linker
+#  -DCMAKE_TOOLCHAIN_FILE=cmake/lld-linux.toolchain.cmake
+
+cd ${builddir} && CC=${CC} CXX=${CXX} cmake \
   -DCMAKE_VERBOSE_MAKEFILE=1 \
   ..