Bring the Z_SOLO define back, just for the zlib.h compat header?

[valpackett]

Some software relies on the fact that #define Z_SOLO would allow using names like compress and uncompress for their own static variables, and because zlib-ng killed the #ifndef Z_SOLO around "utility functions" in the header, boom:

/usr/src/sbin/savecore/savecore.c:108:23: error: redefinition of 'compress' as different kind of symbol
static bool checkfor, compress, uncompress, clear, force, keep; /* flags */
                      ^
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/zlib.h:1205:23: note: previous definition is here
Z_EXTERN int Z_EXPORT compress(unsigned char *dest, unsigned long *destLen, const unsigned char *source, unsigned long sourceLen);
                      ^
/usr/src/sbin/savecore/savecore.c:108:33: error: redefinition of 'uncompress' as different kind of symbol
static bool checkfor, compress, uncompress, clear, force, keep; /* flags */
                                ^
/usr/obj/usr/src/amd64.amd64/tmp/usr/include/zlib.h:1241:23: note: previous definition is here
Z_EXTERN int Z_EXPORT uncompress(unsigned char *dest, unsigned long *destLen, const unsigned char *source, unsigned long sourceLen);
                      ^

Arguably using these names is Not A Good Idea and the software should be changed, but bringing these ifdefs back would improve compatibility.

[mtl1979]

We could do it for compatibility mode, as that is supposed to be backwards compatible with zlib... For non-compatibility mode, we prefix most externally visible functions already...

[nmoinvaz]

This is part of 2.0.3.