build: do not export valgrind with export()

in Seastar, valgrind library is only used at the build time, and is
used internally for accessing its header files when buildings its
internal .cc files. so we don't expect to expose it via `export()`,
for exposing it in `SeastarTargets.cmake`.

but somehow when building Seastar as a static library, valgrind is
still exposed in Seastar's CMake configuration file. if a CMake
based project consume Seastar using its CMake configuration file,
it runs into following failure when CMake generates the building
system:

```
-- Configuring done (2.5s)
CMake Error at external/seastar/build/release/SeastarTargets.cmake:52 (set_target_properties):
  The link interface of target "Seastar::seastar" contains:

    Valgrind::valgrind

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Call Stack (most recent call first):
  external/seastar/build/release/SeastarConfig.cmake:36 (include)
  CMakeLists.txt:6 (find_package)
```

fortunately, in CMake 3.26, `BUILD_LOCAL_INTERFACE` was introduced
this problem in specific, so that the linked library is not included
by `export()` anymore.

in this change, we include `Valgrind::valgrind` using this generator
expression if the CMake's version is greater or equal to 3.26, so that
`Valgrind` is not required for building Seastar applications.

as this generator expression is not available on CMake versions lower
than 3.26, and the minimal required CMake version for building Seastar
is 3.13, and Seastar does support static build. The issue is still not
fully addressed even with this change with older CMake and if Seastar
is built as a static library.

previously, 372a16a intended to address this. but somehow Valgrind
is still exposed by SeastarTargets.cmake.

Refs scylladb#2180
Signed-off-by: Kefu Chai <[email protected]>

CMakeLists.txt

@@ -836,8 +836,17 @@ target_link_libraries (seastar
  rt::rt
  ucontext::ucontext
  yaml-cpp::yaml-cpp
- "$<BUILD_INTERFACE:Valgrind::valgrind>"
  Threads::Threads)
+if (CMAKE_VERSION VERSION_GREATER_EQUAL 3.26)
+ target_link_libraries (seastar
+ PRIVATE
+ "$<BUILD_LOCAL_INTERFACE:Valgrind::valgrind>")
+else ()
+ target_link_libraries (seastar
+ PRIVATE
+ "$<BUILD_INTERFACE:Valgrind::valgrind>")
+endif ()
+
 if (Seastar_DPDK)
  target_link_libraries (seastar
  PRIVATE