Hide the ASTCENC_DECOMPRESSOR option and set it to OFF

It's an option provided by external/astc-encoder/CMakeLists.txt
but setting it to ON in KTX-Software breaks the build.
So set it to OFF and make it an INTERNAL variable so it's not shown
in cmake-gui where it could lure users to enable it.

While at it, also fixed the line number in a comment.

Author: DanielGibson

CMakeLists.txt

@@ -1252,6 +1252,9 @@ if(NOT ${universal_build})
     endif()
 endif()
 
+# setting ASTCENC_DECOMPRESSOR to ON breaks the build, so force ot to OFF
+# and hide it from cmake-gui (by using type INTERNAL)
+set(ASTCENC_DECOMPRESSOR OFF CACHE INTERNAL "")
 set(ASTCENC_CLI OFF) # Only build as library not the CLI astcencoder
 # Force static build for astc-encoder
 set(BUILD_SHARED_LIBS OFF)
@@ -1262,7 +1265,7 @@ target_compile_definitions(
     ${ASTCENC_LIB_TARGET}
 PRIVATE
     # ASTC encoder uses std::mutex. For more info. see comment about
-    # same setting in libktx starting about line 618. To be eventually
+    # same setting in libktx starting about line 633. To be eventually
     # removed as noted in that comment.
     $<$<AND:${is_msvccl},$<VERSION_GREATER_EQUAL:$<CXX_COMPILER_VERSION>,19.40.33811>>:_DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR>
     $<$<AND:${is_clangcl},$<VERSION_GREATER_EQUAL:$<CXX_COMPILER_VERSION>,17.0.3>>:_DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR>