[CI] Build the Windows solution on more than one MSBuild node (#7379)

cmake --build only passes a job count to MSBuild when it is asked for
one, and .ci/compile.sh never asked, so every Visual Studio generator
build ran single-node: projects were built one after another and MTT
only ever parallelized the files of whichever project happened to be
building. Ask for one node per core, overridable with
BUILD_PARALLEL_LEVEL, and fall back to a single node when the core
count is not exposed.

Alongside it set EnforceProcessCountAcrossBuilds and CL_MPCount, the
knobs Microsoft's build throughput write-up pairs with MTT: without
them the node count and MTT multiply into more cl.exe processes than
the runner has cores for.

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

.ci/compile.sh

@@ -18,7 +18,7 @@
 # --cmake-generator <generator> sets CMAKE_GENERATOR as used by cmake
 # --target-macos-version <version> sets the min os version - only used for macOS builds
 # --profile writes an MSBuild performance summary and binary log, Windows only
-# uses env: BUILDTYPE MAKE_INSTALL MAKE_PACKAGE PACKAGE_TYPE PACKAGE_SUFFIX MAKE_SERVER MAKE_NO_CLIENT MAKE_TEST USE_CCACHE CCACHE_SIZE CCACHE_EVICTION_AGE BUILD_DIR CMAKE_GENERATOR TARGET_MACOS_VERSION MAKE_SANITIZE MAKE_COVERAGE VCPKG_TARGET_MACOS_VERSION BUILD_PROFILE
+# uses env: BUILDTYPE MAKE_INSTALL MAKE_PACKAGE PACKAGE_TYPE PACKAGE_SUFFIX MAKE_SERVER MAKE_NO_CLIENT MAKE_TEST USE_CCACHE CCACHE_SIZE CCACHE_EVICTION_AGE BUILD_DIR CMAKE_GENERATOR TARGET_MACOS_VERSION MAKE_SANITIZE MAKE_COVERAGE VCPKG_TARGET_MACOS_VERSION BUILD_PARALLEL_LEVEL BUILD_PROFILE
 # (correspond to args: --debug/--release --install --package <package type> --suffix <suffix> --server --test --sanitize --coverage --ccache <ccache_size> --dir <dir> --profile)
 # exitcode: 1 for failure, 3 for invalid arguments
 
@@ -314,9 +314,20 @@ if [[ $RUNNER_OS == macOS ]]; then
   fi
 
 elif [[ $RUNNER_OS == Windows ]]; then
+  # cmake only hands MSBuild a job count when one is asked for, and without it
+  # MSBuild builds a single node: one project at a time, whatever MTT does inside
+  # it. Ask for a node per core.
+  if [[ ! $BUILD_PARALLEL_LEVEL ]]; then
+    BUILD_PARALLEL_LEVEL="${NUMBER_OF_PROCESSORS:-1}"
+  fi
+  buildflags+=(--parallel "$BUILD_PARALLEL_LEVEL")
+
   # Enable MTT, see https://devblogs.microsoft.com/cppblog/improved-parallelism-in-msbuild/
   # and https://devblogs.microsoft.com/cppblog/cpp-build-throughput-investigation-and-tune-up/#multitooltask-mtt
-  buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true)
+  # EnforceProcessCountAcrossBuilds makes the CL_MPCount ceiling apply across all
+  # the projects the nodes are building, instead of each node running its own
+  # cl.exe processes and /m and MTT multiplying into an oversubscribed machine.
+  buildflags+=(-- -p:UseMultiToolTask=true -p:EnableClServerMode=true -p:EnforceProcessCountAcrossBuilds=true -p:CL_MPCount="$BUILD_PARALLEL_LEVEL")
   if [[ $BUILD_PROFILE ]]; then
     # A performance summary says which projects and tasks cost what, the binary log
     # keeps the full timeline; both are far too noisy to always produce. The files