Fix for CMake < 3.12.0

As several current LTS distributions currently ship with CMake
< 3.12.0, add a work-around for CMake Issue 16967.

Otherwise we get:
CMake Error at
/usr/share/cmake-3.7/Modules/CheckCXXSourceCompiles.cmake:64 (try_compile):
   Unknown extension ".cxx" for file

     /tmp/rtl-sdr/build/CMakeFiles/CMakeTmp/src.cxx

   try_compile() works only for enabled languages.  Currently these are:

     C

CMakeLists.txt

@@ -20,7 +20,13 @@
 # Project setup
 ########################################################################
 cmake_minimum_required(VERSION 3.7.2)
-project(rtlsdr C)
+
+# workaround for https://gitlab.kitware.com/cmake/cmake/issues/16967
+if(${CMAKE_VERSION} VERSION_LESS "3.12.0")
+    project(rtlsdr)
+else()
+    project(rtlsdr C)
+endif()
 
 #select the release build type by default to get optimization flags
 if(NOT CMAKE_BUILD_TYPE)