3rdParty/libpng: `set(PNG_EXECUTABLES OFF)`

Disables executable targets of libpng.

Also, now that CMP0077 is set correctly, we can set
the other options as simple variables.

3rdParty/libpng/CMakeLists.txt

@@ -8,14 +8,16 @@ endif()
 
 if(DEVILUTIONX_STATIC_LIBPNG)
   set(PNG_LIBRARY png_static)
-  set(PNG_STATIC ON CACHE BOOL "Build static lib" FORCE)
-  set(PNG_SHARED OFF CACHE BOOL "Build shared lib" FORCE)
+  set(PNG_STATIC ON)
+  set(PNG_SHARED OFF)
 else()
   set(PNG_LIBRARY png)
-  set(PNG_STATIC OFF CACHE BOOL "Build static lib" FORCE)
-  set(PNG_SHARED ON CACHE BOOL "Build shared lib" FORCE)
+  set(PNG_STATIC OFF)
+  set(PNG_SHARED ON)
 endif()
-set(PNG_TESTS OFF CACHE BOOL "Build libpng tests")
+set(PNG_TESTS OFF)
+set(PNG_EXECUTABLES OFF)
+set(SKIP_INSTALL_PROGRAMS ON)
 
 include(FetchContent)
 FetchContent_Declare(png