fix claimed minimum supported CMake version

fix claimed minimum supported CMake version

Commit 354ed2c644452f300b316d166386bd40d8fec76f, among others, introduced the
use of `$<TARGET_OBJECTS>` in `target_link_libraries` calls. The CMake docs¹
claim this is only officially supported from 3.21 onwards, despite working in
some prior versions. CMake provides binaries going back many versions² so it is
possible to manually bisect exactly what this wording in the CMake docs means.
Running this process yields the answer that CMake 3.9 is the first version that
does not error out when encountering `$<TARGET_OBJECTS>` in a
`target_link_libraries` expression.

Note that the Graphviz CMake files also use some features like `endif` without
an expression, which claims to only be supported from CMake 3.14 onwards.³
Despite this, versions as far back as CMake 3.1 support this syntax just fine.

¹ https://cmake.org/cmake/help/latest/command/target_link_libraries.html#linking-object-libraries-via-target-objects
² https://cmake.org/files/
³ https://cmake.org/cmake/help/v3.13/command/endif.html vs
  https://cmake.org/cmake/help/v3.14/command/endif.html