ci: convert to use CMake

Convert the CI to use CMake instead of Makefiles. This will allow us to
wire up support for Windows in a subsequent commit.

.github/workflows/ci.yml

@@ -19,5 +19,7 @@ jobs:
       uses: actions/checkout@v2
     - name: Build
       run: |
-        cd test
-        make
+        mkdir build
+        cd build
+        cmake ..
+        cmake --build .

test/CMakeLists.txt

@@ -13,8 +13,9 @@ set_target_properties(clar_test PROPERTIES
 	C_EXTENSIONS OFF
 )
 
-# MSVC generates all kinds of warnings.
-if(NOT "${CMAKEC_COMPILER_ID}" MATCHES "MSVC")
+# MSVC generates all kinds of warnings. We may want to fix these in the future
+# and then unconditionally treat warnings as errors.
+if (NOT MSVC)
 	set_target_properties(clar_test PROPERTIES
 		COMPILE_WARNING_AS_ERROR ON
 	)