Set -std=XXX flags earlier; use in AC_COMPILE_IFELSE()

Set -std=XXX flags earlier; use in AC_COMPILE_IFELSE()

Summary:
- We had a bug where AC_COMPILE_IFELSE() invocations weren't using the
same -std=XXX flags that we're using in our actual build. This
happened because we made these checks before figuring out which
flag to use.
- Stop setting -std=XXX flags in CPPFLAGS. It's only useful for
CFLAGS/CXXFLAGS.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: meyering@fb.com

FB internal diff: D1003972