utils/hooks: Fix crash when running from Python build directory.

When running from a Python build directory (without installing), the prefix
for Makefile and config.h is none of the ones tested. So this routine did not
find a possible prefix, leading to an IndexError.

We must not add `dirname(filename)` to the list of prefixes to be tested,
since this would always win the "longest matching prefix" contest.

PyInstaller/utils/hooks/__init__.py

@@ -826,6 +826,9 @@ def _find_prefix(filename):
         common = os.path.commonprefix([prefix, filename])
         if common == prefix:
             possible_prefixes.append(prefix)
+    if not possible_prefixes:
+        # no matching prefix, assume running from build directory
+        possible_prefixes = [os.path.dirname(filename)]
     possible_prefixes.sort(key=lambda p: len(p), reverse=True)
     return possible_prefixes[0]