Fix a bug when using the Library target on Cygwin (1.5.10-3). Include
<unistd.h> for OS_CYGWIN to call the correct version lseek.

The wrong version of lseek is called by file_archscan. The lseek
fails while reading the ar file, causing the routine to read
subsequent bytes instead of seeking to the next ar_hdr entry in the
archive. When the desired object file is not found, jam rebuilds the
entire archive even though the object and proper mtime exists. A
simple test is to just use a single Jamfile and two source
files. Without this fix, jam would relink the second time it was
invoked.

Jamfile:
Library TestLib : file1.c file2.c ;

file1.c:
#define FOO

file2.c:
#define BAR