#21 Test for dirent.h in fileBrowser.c

[Frédéric Fauberteau]

The file fileBrowser.c implements this test:
defined(SYSV) || defined(SVR4) || defined(__CYGWIN__) || defined(__VMS)
but the configure script provides a dedicated test for dirent.h and the macro HAVE_DIRENT_H could be used.
FreeBSD 13.0 does not have sys/dir.h but does not define neither SYSV, SVR4, __CYGWIN__ nor __VMS.
The attached patch fixes the issue.