Because their file name extension is not .so but .dylib, line #25417 of configure script. On my Mac the Qt 3 libraries are installed with Fink in /sw/lib/qt3/lib and as sym-links in the Fink standard directory /sw/lib, C header files can be found in the Fink default directory /sw/include/qt. Mac OS X 10.4.11
This is likely a bug in autoconf that was recently fixed...
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