#14985: Let zlib 1.2.>=7 build on Cygwin.
#15001: Update to prerelease 1.1pre9.
5.11.rc1
Merge 'sage-5.11.rc1' into master
Merge branch 'master' into build_system
[FIXUP] 5.11.rc1: update git specific files

These currently include
 * VERSION.txt
 * package-version.txt
 * checksums.ini