Update curl to 7.55.1

[mickael9]

This is mostly to fix 8ebb600 by rebuilding on trusty (configure script detects the absence of strtok_r) but we might as well update curl.

Built using a clean trusty image with:

./configure --host=x86_64-w64-mingw32 --with-winssl --disable-ldap --disable-file --disable-dict --disable-gopher --disable-imap --disable-imaps --disable-pop3 --disable-rtsp --disable-smb --disable-smtp --disable-telnet --disable-tftp --disable-shared --disable-pthreads
make -j8 -C lib

./configure --host=i686-w64-mingw32 --with-winssl --disable-ldap --disable-file --disable-dict --disable-gopher --disable-imap --disable-imaps --disable-pop3 --disable-rtsp --disable-smb --disable-smtp --disable-telnet --disable-tftp --disable-shared --disable-pthreads
make -j8 -C lib

[zturtleman]

Closing; superseded by #738.