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ARROW-5615: [C++] gcc 5.4.0 doesn't want to parse inline C++11 string R literal

I ran into this while trying to tinker with ARROW-5474 (checking minimum Boost version).

This occurs for me on master with

```
docker-compose build cpp-ubuntu-xenial
docker-compose run cpp-ubuntu-xenial
```

Error looks like

```
/arrow/cpp/src/arrow/ipc/json-simple-test.cc:543:9: error: missing terminating " character [-Werror]
   ASSERT_OK(ArrayFromJSON(type, R"delim(
         ^
/arrow/cpp/src/arrow/ipc/json-simple-test.cc:543:2: error: missing terminating " character
   ASSERT_OK(ArrayFromJSON(type, R"delim(
  ^
/arrow/cpp/src/arrow/ipc/json-simple-test.cc:550:1: error: stray '\' in program
 )delim",
```

I'm perplexed about why this is a problem and why it has not been encountered by others.

Author: Wes McKinney <wesm+git@apache.org>

Closes #4579 from wesm/ARROW-5615 and squashes the following commits:

a4f08fa81 <Wes McKinney> Make parquet-schema-test robust to stripping whitespace
f7ed973bc <Wes McKinney> gcc 5.4.0 doesn't want to parse inline C++11 string R literal