Add configure check for dirent.h and readdir.

[Rob Shearman]

readdir is not available with MSVC. However, it makes sense to still
be able to build crypt32 without the function being present.
---
 configure.ac             |    2 ++
 dlls/crypt32/rootstore.c |    7 +++++++
 2 files changed, 9 insertions(+), 0 deletions(-)
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