Conditionalize libc++ installation

Author: benlangmuir

utils/build-presets.ini

@@ -730,6 +730,7 @@ install-libicu
 install-prefix=/usr
 swift-install-components=autolink-driver;compiler;clang-resource-dir-symlink;stdlib;swift-remote-mirror;sdk-overlay;parser-lib;license;sourcekit-inproc
 llvm-install-components=llvm-cov;llvm-profdata;IndexStore;clang;clang-headers;compiler-rt;clangd
+install-libcxx
 build-swift-static-stdlib
 build-swift-static-sdk-overlay
 build-swift-stdlib-unittest-extra
@@ -1082,6 +1083,7 @@ reconfigure
 
 swift-install-components=compiler;clang-resource-dir-symlink;stdlib;sdk-overlay;parser-lib;license;sourcekit-xpc-service;swift-remote-mirror;swift-remote-mirror-headers
 llvm-install-components=llvm-cov;llvm-profdata;IndexStore;clang;clang-headers;compiler-rt;clangd
+install-libcxx
 
 # Path to the .tar.gz package we would create.
 installable-package=%(installable_package)s

utils/build-script-impl

@@ -207,6 +207,7 @@ KNOWN_SETTINGS=(
     install-swiftevolve         ""               "whether to install the swift-evolve tool"
     install-xctest              ""               "whether to install xctest"
     install-foundation          ""               "whether to install foundation"
+    install-libcxx              ""               "whether to install libc++"
     install-libdispatch         ""               "whether to install libdispatch"
     install-libicu              ""               "whether to install libicu"
     install-playgroundsupport   ""               "whether to install PlaygroundSupport"
@@ -3504,6 +3505,9 @@ for host in "${ALL_HOSTS[@]}"; do
                 INSTALL_TARGETS=install-$(echo ${LLVM_INSTALL_COMPONENTS} | sed -E 's/;/ install-/g')
                 ;;
             libcxx)
+                if [[ -z "${INSTALL_LIBCXX}" ]] ; then
+                    continue
+                fi
                 INSTALL_TARGETS=install-cxx-headers
                 ;;
             swift)