Fix gcc@5.5.0 on OS X 10.13.5 (#8540)

* Fix gcc@5.5.0 on OS X 10.13.5

gcc@5.5.0 failed to build on my OS X 10.13.5 box.

I found/modified a patch in the MacPorts world that traced back to
Homebrewy, https://trac.macports.org/ticket/56502#no1

The issue has also been reported to gcc,
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83531

I eventually discovered a nearly identical version of the patch at
Homebrew.

https://github.com/Homebrew/formula-patches/blob/master/gcc%405/10.13_headers.patch

gcc builds with this and I can compile many things, but e.g. curl and
cmake both fail.  As @davydden and others observed in #1847, it seems
to be an apple thing.

* Fix mistaken change to patch when clause (added :)

I mistakenly deleted a colon from the
'darwin/gcc-7.1.0-headerpad.patch'.