setup.py: if no libevent installation found, go ahead and pretend system-wide libevent is installed

setup.py

@@ -29,9 +29,7 @@ else:
             if os.path.isdir(dir):
                 ev_dir = dir
                 break
-    if not ev_dir:
-        sys.exit("couldn't find libevent installation or build directory")
-
+    if ev_dir:
         print 'found libevent build directory', ev_dir
         ev_incdirs = [ev_dir, ev_dir + '/compat']
         ev_extargs = []
@@ -55,6 +53,11 @@ else:
                              extra_compile_args=ev_extargs,
                              extra_objects=ev_extobjs,
                              libraries=ev_libraries)
+    else:
+        sys.stderr.write("\nWARNING: couldn't find libevent installation or build directory: assuming system-wide libevent is installed.\n\n")
+        libevent = Extension(name=name,
+                             sources=sources,
+                             libraries=['event'])
 
 version = re.search("__version__\s*=\s*'(.*)'", open('gevent/__init__.py').read(), re.M).group(1).strip()
 assert version, version