'LIST_HEAD' macro redefined, built on macOS

[Jim Huang]

The build procedure of dcurl on macOS v10.14:

src/list.h:72:9: warning: 'LIST_HEAD' macro redefined [-Wmacro-redefined]
#define LIST_HEAD(head) struct list_head head = {&(head), &(head)}
        ^
/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/usr/include/sys/queue.h:421:9: note: previous definition is here
#define LIST_HEAD(name, type)                                           \
        ^
1 warning generated.

Macro LIST_HEAD in dcurl conflicts with the headers provided by macOS SDK.

[marktwtn]

Shall we just give the macro another name?