Try to make a sane default for CD detection path by platform.

This is so this can work even when dropping PhysicsFS into another project
 without CMake doing header tests and filling in the defines.

src/platform_unix.c

@@ -21,6 +21,14 @@
 #include <time.h>
 #include <errno.h>
 
+#if PHYSFS_PLATFORM_LINUX && !defined(PHYSFS_HAVE_MNTENT_H)
+#define PHYSFS_HAVE_MNTENT_H 1
+#elif PHYSFS_PLATFORM_SOLARIS && !defined(PHYSFS_HAVE_SYS_MNTTAB_H)
+#define PHYSFS_HAVE_SYS_MNTTAB_H 1
+#elif PHYSFS_PLATFORM_BSD && !defined(PHYSFS_HAVE_SYS_UCRED_H)
+#define PHYSFS_HAVE_SYS_UCRED_H 1
+#endif
+
 #ifdef PHYSFS_HAVE_SYS_UCRED_H
 #  ifdef PHYSFS_HAVE_MNTENT_H
 #    undef PHYSFS_HAVE_MNTENT_H /* don't do both... */