Fix libgit2 build scripts

[phatblat]

I had some trouble getting libgit2 to build in 0.5 and 0.6, getting errors about 'libssh2.h' file not found. I wasn't sure if it was my setup (been on Xcode and Yosemite betas a lot lately), but by modifying the update_libgit2 scripts and defining LIBSSH2_INCLUDE_DIR it builds consistently.

Addresses #442

[jspahrsummers]

This won't be correct for OS X builds, which use the system libssh2 (IIRC).

[jspahrsummers]

Thanks! The iOS changes definitely look good, but I'm a bit unsure about the OS X path.

[phatblat]

Went ahead and reverted the change to the Mac build script. When I run script/update_libgit2 from the command line it builds fine.

It's only when I build the libgit2 target in Xcode that it fails.

[ 88%] Building C object CMakeFiles/git2.dir/src/transports/smart.c.o
[ 89%] Building C object CMakeFiles/git2.dir/src/transports/smart_pkt.c.o
[ 90%] Building C object CMakeFiles/git2.dir/src/transports/smart_protocol.c.o
[ 91%] Building C object CMakeFiles/git2.dir/src/transports/ssh.c.o
/Users/phatblat/dev/ios/APP_NAME/Carthage/Checkouts/objective-git/External/libgit2/src/transports/ssh.c:9:10: fatal error: 'libssh2.h' file not found
#include <libssh2.h>
         ^
1 error generated.
make[2]: *** [CMakeFiles/git2.dir/src/transports/ssh.c.o] Error 1
make[1]: *** [CMakeFiles/git2.dir/all] Error 2
make: *** [all] Error 2

[pietbrauer]

For me both (Mac and iOS) built when I checked out @phatblat branch. Just using the iOS changes didn't solve the building issues.

[jspahrsummers]

Sorry, to be clear: I think LIBSSH2_INCLUDE_DIR is important to pass to the Mac target, I just don't think it should point into the iOS build of libssh2.

Can it use /usr/include or /usr/local/include or something?

[phatblat]

/usr/local/include works

[jspahrsummers]

Awesome, thanks for tackling this! Works great for me. ✨