Clang update.sh: always clear CMake's cache, and only set MACOSX_DEPLOYMENT_TARGET when needed

Previously it was possible to run into situations where CMake would fail,
write a partial CMakeCache.txt, and on the next run of update.sh, we would
think the build was already configured and skip the CMake step.

This patch removes that problem by always clearing the cache and forcing
a full re-run of CMake. This also has the benefit that if the path to the
compiler or a compiler flag changes, the build will get updated.

Note that doing a full re-run of CMake doesn't mean doing a full re-build.

Also, we should only be setting MACOSX_DEPLOYMENT_TARGET when doing bootstrap
builds. This patch fixes that too.

BUG=415670

Review URL: https://codereview.chromium.org/603943003

Cr-Commit-Position: refs/heads/master@{#296771}

tools/clang/scripts/update.sh

@@ -351,19 +351,26 @@ fi
 CFLAGS=""
 CXXFLAGS=""
 LDFLAGS=""
+
 # LLVM uses C++11 starting in llvm 3.5. On Linux, this means libstdc++4.7+ is
 # needed, on OS X it requires libc++. clang only automatically links to libc++
 # when targeting OS X 10.9+, so add stdlib=libc++ explicitly so clang can run on
 # OS X versions as old as 10.7.
 # TODO(thakis): Some bots are still on 10.6, so for now bundle libc++.dylib.
 # Remove this once all bots are on 10.7+, then use --enable-libcpp=yes and
-# change all MACOSX_DEPLOYMENT_TARGET values to 10.7.
+# change deployment_target to 10.7.
+deployment_target=""
+
 if [ "${OS}" = "Darwin" ]; then
   # When building on 10.9, /usr/include usually doesn't exist, and while
   # Xcode's clang automatically sets a sysroot, self-built clangs don't.
   CFLAGS="-isysroot $(xcrun --show-sdk-path)"
   CPPFLAGS="${CFLAGS}"
   CXXFLAGS="-stdlib=libc++ -nostdinc++ -I${ABS_LIBCXX_DIR}/include ${CFLAGS}"
+
+  if [[ -n "${bootstrap}" ]]; then
+    deployment_target=10.6
+  fi
 fi
 
 # Build bootstrap clang if requested.
@@ -447,21 +454,20 @@ if [ "${OS}" = "Darwin" ]; then
   LDFLAGS+="-stdlib=libc++ -L${PWD}/libcxxbuild"
 fi
 
-if [[ ! -f ./CMakeCache.txt ]]; then
-  MACOSX_DEPLOYMENT_TARGET=10.6 cmake -GNinja \
-      -DCMAKE_BUILD_TYPE=Release \
-      -DLLVM_ENABLE_ASSERTIONS=ON \
-      -DLLVM_ENABLE_THREADS=OFF \
-      -DCMAKE_C_COMPILER="${CC}" \
-      -DCMAKE_CXX_COMPILER="${CXX}" \
-      -DCMAKE_C_FLAGS="${CFLAGS}" \
-      -DCMAKE_CXX_FLAGS="${CXXFLAGS}" \
-      -DCMAKE_EXE_LINKER_FLAGS="${LDFLAGS}" \
-      -DCMAKE_SHARED_LINKER_FLAGS="${LDFLAGS}" \
-      -DCMAKE_MODULE_LINKER_FLAGS="${LDFLAGS}" \
-      "${ABS_LLVM_DIR}"
-  env
-fi
+rm -fv CMakeCache.txt
+MACOSX_DEPLOYMENT_TARGET=${deployment_target} cmake -GNinja \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DLLVM_ENABLE_ASSERTIONS=ON \
+    -DLLVM_ENABLE_THREADS=OFF \
+    -DCMAKE_C_COMPILER="${CC}" \
+    -DCMAKE_CXX_COMPILER="${CXX}" \
+    -DCMAKE_C_FLAGS="${CFLAGS}" \
+    -DCMAKE_CXX_FLAGS="${CXXFLAGS}" \
+    -DCMAKE_EXE_LINKER_FLAGS="${LDFLAGS}" \
+    -DCMAKE_SHARED_LINKER_FLAGS="${LDFLAGS}" \
+    -DCMAKE_MODULE_LINKER_FLAGS="${LDFLAGS}" \
+    "${ABS_LLVM_DIR}"
+env
 
 if [[ -n "${gcc_toolchain}" ]]; then
   # Copy in the right stdlibc++.so.6 so clang can start.
@@ -485,7 +491,8 @@ popd
 mkdir -p "${COMPILER_RT_BUILD_DIR}"
 pushd "${COMPILER_RT_BUILD_DIR}"
 
-MACOSX_DEPLOYMENT_TARGET=10.6 cmake -GNinja \
+rm -fv CMakeCache.txt
+MACOSX_DEPLOYMENT_TARGET=${deployment_target} cmake -GNinja \
     -DCMAKE_BUILD_TYPE=Release \
     -DLLVM_ENABLE_ASSERTIONS=ON \
     -DLLVM_ENABLE_THREADS=OFF \
@@ -526,7 +533,8 @@ if [[ -n "${with_android}" ]]; then
   # Build ASan runtime for Android in a separate build tree.
   mkdir -p ${LLVM_BUILD_DIR}/android
   pushd ${LLVM_BUILD_DIR}/android
-  MACOSX_DEPLOYMENT_TARGET=10.6 cmake -GNinja \
+  rm -fv CMakeCache.txt
+  MACOSX_DEPLOYMENT_TARGET=${deployment_target} cmake -GNinja \
       -DCMAKE_BUILD_TYPE=Release \
       -DLLVM_ENABLE_ASSERTIONS=ON \
       -DLLVM_ENABLE_THREADS=OFF \
@@ -552,7 +560,8 @@ TOOL_BUILD_DIR="${ABS_LLVM_BUILD_DIR}/tools/clang/tools/chrome-extras"
 rm -rf "${TOOL_BUILD_DIR}"
 mkdir -p "${TOOL_BUILD_DIR}"
 pushd "${TOOL_BUILD_DIR}"
-MACOSX_DEPLOYMENT_TARGET=10.6 cmake -GNinja  \
+rm -fv CMakeCache.txt
+MACOSX_DEPLOYMENT_TARGET=${deployment_target} cmake -GNinja  \
     -DLLVM_BUILD_DIR="${ABS_LLVM_BUILD_DIR}" \
     -DLLVM_SRC_DIR="${ABS_LLVM_DIR}" \
     -DCMAKE_C_COMPILER="${CC}" \