CMake: Allow building without examples

Building examples with newer GCC encounters the error:

| libp2p_echo_server.cpp: In function 'bool isInsecure(int, char**)':
| libp2p_echo_server.cpp:22:22: error: 'template<class ValueType, int FirstDimension, int ...RestDimensions> class gsl::multi_span' is deprecated [-Werror=deprecated-declarations]
|    22 |     auto args = gsl::multi_span<char *>(argv, argc);
|       |                      ^~~~~~~~~~
| cc1plus: some warnings being treated as errors

TODO: Disable -Werror for examples

Author: eigendude

CMakeLists.txt

@@ -19,6 +19,7 @@ project(libp2p C CXX)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
 option(TESTING "Build tests" ON)
+option(BUILD_EXAMPLES "Build examples" ON)
 option(CLANG_FORMAT "Enable clang-format target" ON)
 option(CLANG_TIDY "Enable clang-tidy checks during compilation" OFF)
 option(COVERAGE "Enable generation of coverage info" OFF)
@@ -91,7 +92,10 @@ include_directories(
 )
 
 add_subdirectory(src)
-add_subdirectory(example)
+
+if (BUILD_EXAMPLES)
+  add_subdirectory(example)
+endif()
 
 if(TESTING)
   enable_testing()