Fix importing TileDB in CMake versions prior to 3.18. (#4671)

#4528 introduced a single `TileDB::tiledb`exported CMake target for
TileDB with either static or dynamic linkage. For compatibility with
previous versions, the targets `TileDB::tiledb_shared` or
`TileDB::tiledb_static` were also defined depending on the linkage, as
`ALIAS`es to `TileDB::tiledb`.

As it turns out however, we cannot use `ALIAS` targets, because they are
always declared in the global scope prior to CMake 3.18 and if
`find_package(TileDB)` is not called in the top-level `CMakeLists.txt`
file, it will fail with `add_library cannot create ALIAS target
"TileDB::tiledb_shared" because target "TileDB::tiledb" is imported but
not globally visible.`.

Nor can we switch to using `IMPORTED INTERFACE` targets and linking them
to `TileDB::tiledb`, because it would bring a minor breaking change[^1].
Because `TileDB::tiledb_shared` would become an `INTERFACE` library, it
does not have an `IMPORTED_LOCATION` anymore, which would cause [calls
to
`install_target_libs(TileDB::tiledb_shared)`](https://github.com/TileDB-Inc/TileDB-VCF/blob/5bcc79b07935ac540c56bf6ed9ee0f5d60bf247e/libtiledbvcf/cmake/Modules/FindTileDB_EP.cmake#L121)
to fail.

Thankfully there is another solution. We set the
[`EXPORT_NAME`](https://cmake.org/cmake/help/latest/prop_tgt/EXPORT_NAME.html)
of the `tiledb` target to either `tiledb_shared` or `tiledb_static`
depending on the linkage, and define `TileDB::tiledb` as an `IMPORTED
INTERFACE` target[^2] to the linkage-specific target. This maintains
full compatibility.

[^1]: Something similar is the "breaking build system change" I talked
about in
#4408 (comment).
After removing the `install_target_libs` calls from this repository, the
change in Curl did not afffect us and we could update much more easily.
[^2]: In this opposite case the unified target _must_ be an `IMPORTED
INTERFACE`. We cannot get the `IMPORTED_LOCATION` of `TileDB::tiledb`,
but since the target is new this is not a breaking change.

---
TYPE: BUILD
DESC: Fix importing TileDB in CMake versions prior to 3.18.

Author: teo-tsirpanis

cmake/inputs/Config.cmake.in

@@ -44,10 +44,14 @@ endif()
 include("${CMAKE_CURRENT_LIST_DIR}/@[email protected]")
 check_required_components("@PROJECT_NAME@")
 
-if(@BUILD_SHARED_LIBS@ AND NOT TARGET TileDB::tiledb_shared) # BUILD_SHARED_LIBS AND NOT TARGET TileDB::tiledb_shared
-  add_library(TileDB::tiledb_shared ALIAS TileDB::tiledb)
-elseif(NOT TARGET TileDB::tiledb_static)
-  add_library(TileDB::tiledb_static ALIAS TileDB::tiledb)
+if(NOT TARGET TileDB::tiledb)
+  if(TARGET TileDB::tiledb_shared)
+    add_library(TileDB::tiledb INTERFACE IMPORTED)
+    set_target_properties(TileDB::tiledb PROPERTIES INTERFACE_LINK_LIBRARIES TileDB::tiledb_shared)
+  elseif(TARGET TileDB::tiledb_static)
+    add_library(TileDB::tiledb INTERFACE IMPORTED)
+    set_target_properties(TileDB::tiledb PROPERTIES INTERFACE_LINK_LIBRARIES TileDB::tiledb_static)
+  endif()
 endif()
 
 # Define a convenience all-caps variable

tiledb/CMakeLists.txt

@@ -835,6 +835,15 @@ endif()
 # the value of the BUILD_SHARED_LIBS variable.
 add_library(tiledb $<TARGET_OBJECTS:TILEDB_CORE_OBJECTS>)
 
+# Export the target as either tiledb_shared or tiledb_static for compatibility.
+# The exported config will create the unified tiledb target that links to either
+# of them.
+if(BUILD_SHARED_LIBS)
+  set_target_properties(tiledb PROPERTIES EXPORT_NAME tiledb_shared)
+else()
+  set_target_properties(tiledb PROPERTIES EXPORT_NAME tiledb_static)
+endif()
+
 file(READ "${CMAKE_CURRENT_SOURCE_DIR}/sm/c_api/tiledb_version.h" ver)
 
 string(REGEX MATCH "TILEDB_VERSION_MAJOR ([0-9]*)" _ ${ver})