Build-system fixes (#606)

* CMake: Add path to dfdutils headers for toktx

That path was being added to the include dirs of the library ktx only
when `KTX_FEATURE_VULKAN` was enabled, resulting in a build failure
otherwise:

```
    /home/pmoreau/Softwares/KTX-Software/tools/toktx/pngimage.cc:26:10: fatal error: 'dfd.h' file not found
    #include "dfd.h"
             ^~~~~~~
    1 error generated.
```

* CMake: Only build *loadtests if corresponding loader enabled

vkloadtests was still being built even if `KTX_FEATURE_VULKAN` was
disabled, resulting in missing symbols when loading against the ktx
library.

tests/loadtests/CMakeLists.txt

@@ -196,9 +196,11 @@ set( LOAD_TEST_COMMON_RESOURCE_FILES
     ${MODELS}
 )
 
-include(glloadtests.cmake)
+if(KTX_FEATURE_GL_UPLOAD)
+    include(glloadtests.cmake)
+endif()
 
-if(NOT EMSCRIPTEN)
+if(KTX_FEATURE_VULKAN AND NOT EMSCRIPTEN)
     include(vkloadtests.cmake)
 endif()
 

tools/toktx/CMakeLists.txt

@@ -23,6 +23,7 @@ PRIVATE
     $<TARGET_PROPERTY:objUtil,INTERFACE_INCLUDE_DIRECTORIES>
     ${PROJECT_SOURCE_DIR}/lib
     ${PROJECT_SOURCE_DIR}/lib/basisu
+    ${PROJECT_SOURCE_DIR}/lib/dfdutils
     ${PROJECT_SOURCE_DIR}/other_include
 )