Update vendored gtest to 1.12.1

[falbrechtskirchinger]

Update googletest to the last version supporting C++11.

This is required to compile with -Wexit-time-destructors.

Steps to verify:

• Download gtest v1.12.1 from here: https://github.com/google/googletest/releases/tag/release-1.12.1
• Unpack and cd into googletest directory.
• Run: find src include -type f -exec md5sum {} \; >sums
• Clone this PR: git clone --depth 1 --branch update-gtest https://github.com/falbrechtskirchinger/cpp-httplib.git
• cd into the test/gtest directory.
• Run: md5sum -c path/to/sums; echo $?
• The exit code should be 0.

[yhirose]

Great!