Default to MAC_OS_X_VERSION_MIN_REQUIRED=1080 for Mac OS X SDK >= 10.8

...esp. after aba0106926ceb337035cf46611ff0d590a2af28b "Always build 64-bit code
for OS X" combined with the fact that the TDF 64-bit builds state 10.8 as a
minimum requirement.

For a build with SDK >= 10.8 and no explicit --with-macosx-version-min-required,
this also enforces libc++ instead of libstdc++, which is a prerequisite for
future use of more C++11 features.

Change-Id: Idc770c30ff844b6c3e84d088b5deead0a577e7bf