This commit is partial effort toward more libsoxr integration, that implements Rob Sykes's latest patch. I have no idea whether the Linux-specific stuff from the patch (e.g., M4 file) is still correct in this.

For Resample.* and QualityPrefs.cpp, this commit has my restructuring for distinguishing constant-rate vs variable-rate resamplers more generally. I think it's complete and ready for const-rate, but I have more review and testing to do for the var-rate cases. 

Variable-rate resampling is not implemented here, so Time Tracks are still broken, but this is a milestone in getting to a more general and correct structure. 

Also I think this fixes AboutDialog issues Steve noticed.

m4/audacity_checklib_libresample.m4

@@ -0,0 +1,46 @@
+dnl Add Audacity license?
+dnl Please increment the serial number below whenever you alter this macro
+dnl for the benefit of automatic macro update systems
+# audacity_checklib_libresample.m4 serial 1
+
+AC_DEFUN([AUDACITY_CHECKLIB_LIBRESAMPLE], [
+
+   AC_ARG_WITH(libresample,
+               [AS_HELP_STRING([--with-libresample],
+                               [use libresample for sample rate conversion: [yes,no]])],
+               LIBRESAMPLE_ARGUMENT=$withval,
+               LIBRESAMPLE_ARGUMENT="unspecified")
+
+   if false ; then
+      AC_DEFINE(USE_LIBRESAMPLE, 1,
+                [Define if libresample support should be enabled])
+   fi
+
+   dnl see if libresample is installed on the system
+
+   dnl ... but libresample isn't generally installed as a system library...
+
+   LIBRESAMPLE_SYSTEM_AVAILABLE="no"
+
+   dnl see if libresample is available locally
+
+   AC_CHECK_FILE(${srcdir}/lib-src/libresample/include/libresample.h,
+                 resample_h_found="yes",
+                 resample_h_found="no")
+
+   if test "x$resample_h_found" = "xyes" ; then
+      LIBRESAMPLE_LOCAL_AVAILABLE="yes"
+      LIBRESAMPLE_LOCAL_LIBS="libresample.a"
+      LIBRESAMPLE_LOCAL_CXXFLAGS='-I$(top_srcdir)/lib-src/libresample/include'
+      LIBRESAMPLE_LOCAL_CPPSYMBOLS="USE_LIBRESAMPLE"
+
+      if test ! -f lib-src/libresample/Makefile ; then
+         LIBRESAMPLE_LOCAL_CONFIG_SUBDIRS="lib-src/libresample"
+      fi
+      AC_MSG_NOTICE([libresample libraries are available in the local tree])
+   else
+      LIBRESAMPLE_LOCAL_AVAILABLE="no"
+      AC_MSG_NOTICE([libresample libraries are NOT available in the local tree])
+   fi
+])
+