setup.py: don't error out when PIL can't be found

It may still be in the include directories provided by the
operating system, such as it is on the vast majority of Linux
distributions.

setup.py

@@ -213,11 +213,6 @@ if not os.path.exists(imaging_path):
 if os.path.exists(imaging_path):
     pil_include.append(imaging_path)
 
-if not pil_include:
-    print('ERROR: Could not find Python Imaging library')
-    print(BUILD_ERROR)
-    sys.exit(1)
-
 SRC_PATH = 'overviewer_core/src/'
 
 # used to figure out what files to compile