FreeBSD/src 1109b77 (r346574)lib/libc++ Makefile, lib/libc++experimental Makefile

Rework CXXSTD setting via r345708

This change allows the user to once again override the C++ standard, restoring
high-level pre-r345708 behavior.

This also unbreaks building lib/ofed/libibnetdisc/Makefile with a non-C++11
capable compiler, e.g., g++ 4.2.1, as the library supported being built with
older C++ standards.

MFC after:      2 weeks
MFC with:       r345708
Reviewed by:    emaste
Reported by:    jbeich
Differential Revision: https://reviews.freebsd.org/D19895 (as part of a larger change)
DeltaFile
+3-0lib/ofed/libibnetdisc/Makefile
+1-1lib/libc++experimental/Makefile
+1-1lib/libc++fs/Makefile
+1-1lib/libc++/Makefile
+1-1lib/libgcc_eh/Makefile.inc
+2-0usr.bin/dtc/Makefile
+1-1lib/libcxxrt/Makefile
+10-57 files

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