Explicitly use at least C++17 for compilation

Signed-off-by: Tin Švagelj <tin.svagelj@live.com>

Author: Caellian

src/CMakeLists.txt

@@ -407,6 +407,7 @@ endif(BUILD_INTEL_BACKLIGHT)
 if(BUILD_TESTS)
   # Create a library strictly for testing
   add_library(conky_core ${conky_sources} ${optional_sources})
+  target_compile_features(conky_core PUBLIC cxx_std_17)
   add_dependencies(conky_core generated_hdr_files)
   target_link_libraries(conky_core ${conky_libs})
   add_executable(conky main.cc)
@@ -418,6 +419,7 @@ if(BUILD_TESTS)
     ARCHIVE DESTINATION ${LIB_INSTALL_DIR})
 else()
   add_executable(conky main.cc ${conky_sources} ${optional_sources})
+  target_compile_features(conky_core PRIVATE cxx_std_17)
   add_dependencies(conky generated_hdr_files)
   target_link_libraries(conky ${conky_libs})
 endif()

tests/CMakeLists.txt

@@ -27,6 +27,7 @@ file(GLOB mock_sources mock/*.cc)
 add_library(Catch2 STATIC catch2/catch_amalgamated.cpp)
 
 add_executable(test-conky test-common.cc ${test_sources})
+target_compile_features(test-conky PRIVATE cxx_std_17)
 target_include_directories(test-conky
   PUBLIC
   ${CMAKE_SOURCE_DIR}/src