Compiling _macosx.m on Lion gives 3 warnings:

running build_ext
gcc-4.2 not found, using clang instead
building 'matplotlib.backends._macosx' extension
clang -fno-strict-aliasing -fno-common -dynamic -arch i386 -arch x86_64 -g -O2 -DNDEBUG -g -O3 -DPY_ARRAY_UNIQUE_SYMBOL=MPL_ARRAY_API -DPYCXX_ISO_CPP_LIB=1 -I/usr/local/include -I/usr/include -I/usr/X11/include -I/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/numpy/core/include -I/usr/local/include -I/usr/include -I/usr/X11/include -I. -I/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/numpy/core/include -Isrc -Iagg24/include -I. -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/_macosx.m -o build/temp.macosx-10.6-intel-2.7/src/_macosx.o
src/_macosx.m:3808:26: warning: sending 'View *' to parameter of incompatible type
      'id'
    [window setDelegate: view];
                         ^~~~
/System/Library/Frameworks/AppKit.framework/Headers/NSWindow.h:308:44: note: passing
      argument to parameter 'anObject' here
- (void)setDelegate:(id )anObject;
                                           ^
src/_macosx.m:4253:20: warning: incompatible pointer types assigning to
      'ScrollableButton *' from 'NSButton *' [-Wincompatible-pointer-types]
            button = [NSButton alloc];
                   ^ ~~~~~~~~~~~~~~~~
/System/Library/Frameworks/Foundation.framework/Headers/NSObject.h:76:1: note: class
      method 'alloc' is assumed to return an instance of its receiver type ('NSButton *')
+ (id)alloc;
^
src/_macosx.m:4932:31: warning: 'filename' is deprecated [-Wdeprecated-declarations]
        NSString* filename = [panel filename];
                              ^
3 warnings generated.

Although these may be harmless, it would be nicer if they could be avoided entirely.