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This fixes the linux/gcc error building Google Test with gcc 4.7:

https://travis-ci.org/github/jbeder/yaml-cpp/jobs/668233706

* Showing test output only on failure

* Test commit to see the failed test

* Now making sure that the tests pass with no output

Remove 2.6-isms
Remove 2.8-isms
Bump CMake minimum version to 3.4

Disable some options when used as a subdirectory

Use `CONFIGURE_DEPENDS` with `file(GLOB)` when possible

Backport CMake 3.15's MSVC_RUNTIME_LIBRARY setting.
Set all compile options as generator expressions.
Set all find-package files to be installed to the correct file.

Remove `export(PACKAGE)`, as this has been deprecated.
Remove fat binary support
Remove manual setting of iPhone settings. These should be set by parent
projects.
Remove use of ExternalProject for a local use
Conditionally remove format target unless clang-format is found

The combination of clang++ 3.4 with libstdc++ from GCC 4.6
doesn't work.

Apparently the default is

on_success: change
on_failure: always

which seems reasonable.

Hooray for testing travis configs via pushing to master.

gcc 4.6 (the default) does have some C++11 support, but its flag is -std=c++0x. Since 4.7 was released in 2012, I think it's reasonable to require it.