Error out if tar can't be found.

Only applies for platforms other than Windows.

bitmaps_png/CMakeLists.txt

@@ -749,11 +749,15 @@ endfunction()
 
 include( FindUnixCommands )
 
-# CMake on Windows CI seems to struggle to find tar even though it's in PATH
-# Let's help it out as modern windows (Server 2019 and 10 provide tar)
-if( MSVC AND TAR STREQUAL "TAR-NOTFOUND" )
-   # yolo
-    set( TAR "C:\\Windows\\System32\\tar.exe" )
+if( TAR STREQUAL "TAR-NOTFOUND" )
+    # CMake on Windows CI seems to struggle to find tar even though it's in PATH
+    # Let's help it out as modern windows (Server 2019 and 10 provide tar)
+    if( MSVC )
+        # yolo
+        set( TAR "C:\\Windows\\System32\\tar.exe" )
+    else()
+        message( FATAL_ERROR "Could not find the tar program." )
+    endif()
 endif()
 
 set( BITMAP_ARCHIVE_PATH ${CMAKE_BINARY_DIR}/resources/images.tar.gz )