Add SYSTEM option (#449)

* add system property for cpm_add_subdirectory

* add test case for system property

* lock CMake version in test workflow

* refactor to make SYSTEM an extra config option and update tests

* remove old comment change

* use consistent CMake version and extension for all workflows

* make warning more specific and try to trigger on windows

* another attempt to trigger warning on MSVC

* update readme

* simplify test case and use git tag

* add SYSTEM option to .cmake-format

* forward system arg for source overrides

* enable system implicitly for the single argument syntax

* Use SYSTEM option for FetchContent and add_subdirectory (#441)

* Use SYSTEM option for FetchContent and add_subdirectory

* Add SYSTEM option to syntax and doku

* Update CPM.cmake

* Update .cmake-format

---------

Co-authored-by: Lars Melchior <TheLartians@users.noreply.github.com>

---------

Co-authored-by: Claus Klein <claus.klein@arcormail.de>

.github/workflows/style.yaml

@@ -0,0 +1,31 @@
+name: Style
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  style:
+
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Setup cmake
+      uses: jwlawson/actions-setup-cmake@v1.13
+      with:
+        cmake-version: '3.25.x'
+    
+    - name: Install format dependencies
+      run: pip3 install clang-format==14.0.6 cmake_format==0.6.11 pyyaml
+
+    - name: configure
+      run: cmake -Stest/style -Bbuild/style
+
+    - name: check style
+      run: cmake --build build/style --target check-format