build-llvm: Use llvm-libtool-darwin if doing LTO macOS builds with a custom clang

build-llvm.sh

@@ -298,6 +298,19 @@ if [ -z "$HOST" ] && [ "$(uname)" = "Darwin" ]; then
         # This silences a cmake warning.
         CMAKEFLAGS="$CMAKEFLAGS -DLLDB_USE_SYSTEM_DEBUGSERVER=ON"
     fi
+    if [ -n "$WITH_CLANG" ] && [ -n "$LTO" ]; then
+        # If doing LTO, we need to make sure other related tools are used.
+        # CMAKE_LIBTOOL is not a standard cmake tool, but an LLVM specific
+        # thing. It defaults to looking up the tool with xcrun rather than
+        # looking in paths first.
+        # If building for multiple architectures at once
+        # (CMAKE_OSX_ARCHITECTURES), we also need to provide a tool named
+        # "lipo" in the PATH (this is invoked by Clang directly, so we can't
+        # specify it here unless we pass in the option "-fuse-lipo="). If
+        # the LLVM CMake build wasn't using libtool, we would also need to
+        # specify LLVM_AR and LLVM_RANLIB.
+        CMAKEFLAGS="$CMAKEFLAGS -DCMAKE_LIBTOOL=$(command -v llvm-libtool-darwin)"
+    fi
 fi
 
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