iterm2: fix build

During iterm2's build, xcodebuild invokes $LD and passes it options such
as -isysroot. These options are intended for the linker driver (clang),
not for the linker directly. ld64 ($LD) does not recognize these
options, causing iterm2's build to fail.

Set $LD to $CC (clang) as intended, making iterm2's build succeed.