I think this is caused by a bug in ncurses.
The ncmpcpp build system uses ncursesw5-config to get ncurses specific link flags.
while it should output
Curl uses the linker flag -lcurl. Libtool combines the flags -L/usr/lib and -lcurl to an absolute path /usr/lib/libcurl.so which is the 32-bit library.
If I modify /usr/bin/ncursesw5-config to output the correct flags, ncmpcpp compiles successfully.
It seems the bug has been fixed in latest ncurses snapshots.