simple install: use 'cc', not 'gcc'

Use 'cc' as the default compiler in the name of increased portability.

https://bugzilla.gnome.org/show_bug.cgi?id=720119
sysdeps: grok -isystem

sysdeps inspects the user's CFLAGS and CPPFLAGS to try to get a list of
system directories in which to search for c_include-type sysdeps.

It does this by looking for -I flags.  Make sure it also includes
-isystem.

https://bugzilla.gnome.org/show_bug.cgi?id=720083
bootstrap: get autotools as .tar.gz, not .tar.xz

On Mac OS, we have none of libtool automake or xz in the base system.
xz depends on automake and automake depends on libtool, so we need to
download libtool and automake as .tar.gz to break the cycle.

Also: use http instead of ftp, as it's more reliable.

https://bugzilla.gnome.org/show_bug.cgi?id=720285