zlib dependency wrong

[asedeno]

I tried to build curl 7.60.0 on SunOS 5.10 on sparc. The system zlib.h version is 1.1.4.

https://github.com/curl/curl/blob/master/docs/INTERNALS.md#dependencies says that's good enough

4acc9d3 started using Z_BLOCK instead of Z_SYNC_FLUSH when calling inflate() in lib/content_encoding.c.

https://github.com/madler/zlib/blob/master/ChangeLog#L845-L850 says that Z_BLOCK was added in zlib 1.2.0.5

Z_BLOCK should be tested for, or the dependencies should be updated. I'd prefer the former, as I'm not going to get to update the system zlib here.

SunOS 5.10 Generic_118833-36 sun4u sparc SUNW,UltraAX-i2