Fix user needs to set C++17 on their target to link to Adiar

docs/getting_started.md

@@ -57,12 +57,8 @@ _CMakeLists.txt_ file with the following lines.
 ```cmake
 add_executable(<target> <source>)
 target_link_libraries(<target> adiar)
-set_target_properties(<target> PROPERTIES CXX_STANDARD 17)
 ```
 
-You only need to include the third line if the `CXX_STANDARD` has not been set
-project-wide to 17 or higher.
-
 ## Usage
 After having linked the C++ source file with _Adiar_ as described above, then
 one needs to include the `<adiar/adiar.h>` header, initialise the library before

src/adiar/CMakeLists.txt

@@ -98,6 +98,8 @@ set_target_properties(adiar PROPERTIES
   POSITION_INDEPENDENT_CODE ON
 )
 
+target_compile_features(adiar PUBLIC cxx_std_17)
+
 install(TARGETS adiar
         ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
         LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}