ASACORE-1222 Use DEF file to export C APIs.

Make the alljoyn_c exports more consistent with typical C Windows SDK
exports, by exporting using a DEF file instead of dllexport.
Windows SDK tool chain prefers DEF-style export names - without the
starting '_' character and trailing '@N' characters added by
dllexport on x86.

Change-Id: I81fa98c93b4884db2f8fd20f6bde495c4a347e9b
Signed-off-by: Daniel Mihai <Daniel.Mihai@microsoft.com>
Merge "ASACORE-1222 Use DEF file to export C APIs." into RB14.12