Added C language to the base project

The Cmake check IPO script seems to need it in order to do its
initial checks on all compilers used in the project.
If you add source files that it did not check beforehand it
complains...

CMakeLists.txt

@@ -1,5 +1,5 @@
 cmake_minimum_required(VERSION 3.13)
-project (sfizz VERSION 1.0.0 LANGUAGES CXX)
+project (sfizz VERSION 1.0.0 LANGUAGES CXX C)
 
 # External configuration CMake scripts
 set (CMAKE_MODULE_PATH "${CMAKE_MODULE_PATH};${CMAKE_CURRENT_SOURCE_DIR}/cmake")

lv2/CMakeLists.txt

@@ -20,7 +20,6 @@ set (LV2PLUGIN_TTL_SRC_FILES
     ${PROJECT_NAME}.ttl.in
 )
 add_library (${LV2PLUGIN_PRJ_NAME} SHARED ${PROJECT_NAME}.c ${LV2PLUGIN_TTL_SRC_FILES})
-set_source_files_properties(${PROJECT_NAME}.c LANGUAGE CXX)
 target_link_libraries (${LV2PLUGIN_PRJ_NAME} ${PROJECT_NAME}::${PROJECT_NAME})
 
 # Fetch required LV2 include files