build(cmake): always mark HAS_STRING_VIEW as part…

…of the interface

If the library gets compiled with HAS_STRING_VIEW=1, consumers always
need to link to the functions using std::string_view, as they are the
only ones compiled into the shared library.

You can find a longer explanation here:
<#754 (comment)>

CMakeLists.txt

@@ -100,6 +100,8 @@ else()
 endif()
 if ( DISABLE_STRING_VIEW )
   target_compile_definitions( date INTERFACE HAS_STRING_VIEW=0 -DHAS_DEDUCTION_GUIDES=0 )
+else()
+  target_compile_definitions( date INTERFACE HAS_STRING_VIEW=1 )
 endif()
 
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