[libc++] Fix local runs of the bootstrapping-build job

The bootstrapping-build job defined in run-buildbot required the
CC and CXX environment variables to be defined even though run-buildbot
documents these environment variables as being optional.

It also required ccache, which may not be available for local builds.

This patch tweaks the script slightly to better deal with the absence
of these environment variables/tools.

Author: ldionne

libcxx/utils/ci/run-buildbot

@@ -96,6 +96,14 @@ if [ -z "${CMAKE}" ]; then
     fi
 fi
 
+if [ -z "${CC}" ]; then
+    CC=cc
+fi
+
+if [ -z "${CXX}" ]; then
+    CXX=c++
+fi
+
 function step() {
   endstep
   set +x
@@ -121,6 +129,13 @@ function error() {
     echo "::error::$1"
 }
 
+# Print the version of a few tools to aid diagnostics in some cases
+step "Diagnose tools in use"
+${CMAKE} --version
+${NINJA} --version
+${CC} --version
+${CXX} --version
+
 function clean() {
     rm -rf "${BUILD_DIR}"
 }
@@ -240,12 +255,6 @@ function test-armv7m-picolibc() {
     check-runtimes
 }
 
-# Print the version of a few tools to aid diagnostics in some cases
-step "Diagnose tools in use"
-${CMAKE} --version
-${NINJA} --version
-if [ ! -z "${CXX}" ]; then ${CXX} --version; fi
-
 case "${BUILDER}" in
 check-generated-output)
     # `! foo` doesn't work properly with `set -e`, use `! foo || false` instead.
@@ -382,12 +391,16 @@ generic-ubsan)
 bootstrapping-build)
     clean
 
+    if which ccache; then
+        CCACHE="-DCMAKE_CXX_COMPILER_LAUNCHER=ccache"
+    fi
+
     step "Generating CMake"
     ${CMAKE} \
           -S "${MONOREPO_ROOT}/llvm" \
           -B "${BUILD_DIR}" \
           -GNinja -DCMAKE_MAKE_PROGRAM="${NINJA}" \
-          -DCMAKE_CXX_COMPILER_LAUNCHER="ccache" \
+          ${CCACHE} \
           -DCMAKE_BUILD_TYPE=Release \
           -DCMAKE_INSTALL_PREFIX="${INSTALL_DIR}" \
           -DLLVM_ENABLE_PROJECTS="clang;lldb" \