Log Message:
zlib: always include unistd.h on Linux

Newer compilers, such as gcc14 in Fedora 40, make calling undeclared
functions an error. zlib uses lseek, which is in unistd.h. The existing
zconf.h has complex logic to include unistd.h in some cases, when it should
probably just include it unconditionally.

Set -DZ_HAVE_UNISTD_H in CFLAGS.Linux. This should probably just be added
unconditionally, but I was hesitant to do so without asking.