pack_wheel: Work on OS X with SIP

The old DYLD_LIBRARY_PATH tricks don't work any more (at least, not
while System Integrity Protection is on). Even just getting that into
os.environ seems difficult on recent OS X!

    % DYLD_LIBRARY_PATH=$HOME/local/lib python -c \
      'import os; print(os.environ.get("DYLD_LIBRARY_PATH"))'
    None

LDFLAGS still get passed around OK, though, so go looking for extra
library search paths in them. Presumably there's something like

    -L$HOME/local/lib

in there if the developer needs it.

Signed-off-by: Tim Burke <tim.burke@gmail.com>
Change-Id: I85f7a1b4308e01858bff5fccda1f5a4b54957da8

pack_wheel.py

@@ -72,6 +72,16 @@ def locate_library(name, missing_ok=False):
     if libpath:
         for d in libpath.split(':'):
             cmd.extend(['-L', d.rstrip('/')])
+    should_read_path = False
+    for flg in os.environ.get('LDFLAGS', '').split(' '):
+        if should_read_path:
+            cmd.append(flg)
+            should_read_path = False
+        elif flg == '-L':
+            cmd.append(flg)
+            should_read_path = True
+        elif flg.startswith('-L'):
+            cmd.append(flg)
     cmd.extend(['-o', os.devnull, '-l%s' % name])
     p = subprocess.run(cmd, capture_output=True, text=True)
     # Note that we don't expect the command to exit cleanly; we just want