Updates to mkdist.sh to run under Yosemite with Xcode 6

[ryangenz]

To get the latest version 20131216 to run under Yosemite (10.10) with Xcode 6, only a few tweaks to mkdist.sh are required.

In mkdist.sh, around line 50 replace:
sysroot="$xcodepath/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk"
with:
sysroot="$xcodepath/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk"

It does not like the GMP FTP path anymore, but it'll work fine with the HTTP path-
so near line 462, replace:
getPackage http://ftp.gmplib.org/gmp/gmp-"$version_gmp".tar.bz2
with
getPackage http://gmplib.org/download/gmp/gmp-"$version_gmp".tar.bz2

(You may want to update to more recent package versions as well, but that should probably be in a different 'Issues' thread.)

[q2dg]

Could this be merged as a PR??