configure: add additional flags to LDFLAGS on OS/2

This allows to build a DLL and to open files in binary mode.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
src/dvd_read.c: fix a compilation warning on OS/2

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src/dvd_reader.c: In function `DVDOpen':
src/dvd_reader.c:561: warning: implicit declaration of function `isalpha'
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Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>