Build-system fixes

[pierremoreau]

• Fix missing header when building toktx while KTX_FEATURE_VULKAN is disabled.
• Do not build glloadtests and vkloadtests when their corresponding loader is not enabled, otherwise it results in missing symbols. For example, do not build vkloadtests when KTX_FEATURE_LOADTEST_APPS is enabled but KTX_FEATURE_VULKAN is not.

[CLAassistant]

All committers have signed the CLA.

[MarkCallow]

I see pngimage.cc includes dfd.h. How come it managed to be found when KTX_FEATURE_VULKAN was enabled?

[pierremoreau]

I see pngimage.cc includes dfd.h. How come it managed to be found when KTX_FEATURE_VULKAN was enabled?

That is due to:

1. The include directories for libktx being modified to include the following when KTX_FEATURE_VULKAN is enabled:

       target_include_directories(
           ${lib}
       PRIVATE
           $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/lib/dfdutils>
           $<INSTALL_INTERFACE:lib/dfdutils>
       )

2. And toktx adding the following to its own include directories:

   $<TARGET_PROPERTY:ktx,INCLUDE_DIRECTORIES>

[MarkCallow]

Thank you for the explanation and for this fix.

[pierremoreau]

No worries! Thank you for the quick replies and work on the project.