Fix OpenJPEG target handling in DCMTKConfig.cmake

[lassoan]

When DCMTK is built with OpenJPEG support, DCMTKTargets.cmake contains
INTERFACE_LINK_LIBRARIES entries that reference the imported target
openjp2. If a consuming project includes DCMTKConfig.cmake without
previously finding OpenJPEG, CMake errors out because the openjp2
target does not exist.

Use CMakeFindDependencyMacro to conditionally call find_dependency(OpenJPEG)
when DCMTK_WITH_OPENJPEG is enabled. This guarantees that the openjp2
target is defined before DCMTKTargets.cmake is processed,
while avoiding redundant lookups in projects that have already found
OpenJPEG themselves.

[michaelonken]

@eichelberg This is probably something for you.

[Copilot]

Pull request overview

This PR fixes a CMake package-consumption issue when DCMTK is built with OpenJPEG support: importing DCMTKTargets.cmake can fail if the consuming project hasn’t already created the openjp2 imported target.

Changes:

• Add a guarded find_package(OpenJPEG ...) call in DCMTKConfig.cmake generation so openjp2 is defined before importing DCMTK targets.
• Only perform the OpenJPEG lookup when DCMTK was built with OpenJPEG and the openjp2 target does not already exist.

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[Copilot]

find_package(OpenJPEG CONFIG QUIET) suppresses why OpenJPEG wasn’t found; if this call fails, include(DCMTKTargets.cmake) will still error later due to a missing openjp2 target, but with a less actionable message. Consider using REQUIRED or checking the result and emitting a clearer FATAL_ERROR that suggests setting OpenJPEG_DIR when DCMTK was built with OpenJPEG support.

Suggested change

	find_package(OpenJPEG CONFIG QUIET)
	find_package(OpenJPEG CONFIG QUIET)
	if(NOT TARGET openjp2)
	message(FATAL_ERROR
	"DCMTK was built with OpenJPEG support, but the OpenJPEG package "
	"configuration could not be found. Please make OpenJPEG available to "
	"CMake, for example by setting OpenJPEG_DIR to the directory "
	"containing OpenJPEGConfig.cmake or by adjusting CMAKE_PREFIX_PATH.")
	endif()

[lassoan]

@eichelberg please have a look. Without this, downstream projects need to add find_package calls depending on what DCMTK modules are enabled.

Note that the same problem is fixed for OpenSSL and zlib packages here: #67