build: Add CMake-based build system

Co-authored-by: Tim Ruffing <crypto@timruffing.de>

cmake/TryAddCompileOption.cmake

@@ -0,0 +1,23 @@
+include(CheckCCompilerFlag)
+
+function(try_add_compile_option option)
+  string(MAKE_C_IDENTIFIER ${option} result)
+  string(TOUPPER ${result} result)
+  set(result "C_SUPPORTS${result}")
+  set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+  if(NOT MSVC)
+    set(CMAKE_REQUIRED_FLAGS "-Werror")
+  endif()
+  check_c_compiler_flag(${option} ${result})
+  if(${result})
+    get_property(compile_options
+      DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+      PROPERTY COMPILE_OPTIONS
+    )
+    list(APPEND compile_options "${option}")
+    set_property(
+      DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
+      PROPERTY COMPILE_OPTIONS "${compile_options}"
+    )
+  endif()
+endfunction()