Mark mbusd as a pure C project (#109)

Otherwise cmake configuration might fail if there is no C++ compiler:

CMake Error at CMakeLists.txt:3 (project):
  No CMAKE_CXX_COMPILER could be found.

  Tell CMake where to find the compiler by setting either the environment
  variable "CXX" or the CMake cache entry CMAKE_CXX_COMPILER to the full path
  to the compiler, or to the compiler name if it is in the PATH.

CMakeLists.txt

@@ -1,6 +1,6 @@
 cmake_minimum_required(VERSION 3.2)
 
-project(mbusd VERSION 0.5.2)
+project(mbusd VERSION 0.5.2 LANGUAGES C)
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/extern_GPL)
 include(CheckFunctionExists)