[patch] libstdc++/66018 avoid AC_SEARCH_LIBS test

[Jonathan Wakely]

The AC_STRUCT_DIRENT_D_TYPE autoconf macro expands to include
AC_SEARCH_LIBS for opendir, which can't be used when cross-compiling.
We don't actually care about crufty old systems that put opendir()
somewhere weird, so this just replaces AC_STRUCT_DIRENT_D_TYPE with a
manual check for the part we care about.

Tested powerpc64le-linux, committed to trunk.
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