Add missing include in CMakeLists

Add the `include(CheckSymbolExists)` to allow the usage of the `check_symbol_exists()` macro and fix #136

CMakeLists.txt

@@ -23,6 +23,7 @@ else()
 endif()
 
 include(GNUInstallDirs)
+include(CheckSymbolExists)
 add_subdirectory(lib)
 
 if(BUILD_EXAMPLES)