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/examples.yaml

@@ -0,0 +1,30 @@
+name: Examples
+
+on:
+  push:
+    branches:
+    - master
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  gcc:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: build all
+        env:
+          CC: gcc
+          CXX: g++
+        run: python3 examples/build_all.py
+
+  clang:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: build all
+        env:
+          CC: clang
+          CXX: clang++
+        run: python3 examples/build_all.py