Fix jemalloc stats under perfschema builds

Summary:
Mark found this build error, and just disabled building this unittest.

This reverts that diff, and adds a more correct fix in its place.

This adds MYSQLD_LDFLAGS, if present, to this unittest's linking step.

Test Plan:
mysqlbuild.sh --perfschema --clean
mysqlbuild.sh --clean
cmake && make

Reviewers: santoshb

Reviewed By: santoshb
Fix a few build warnings.

Summary:
Some warnings are hidden by the build system, so we didn't see these.

Now we run all our builds in maintainer mode (-Werror).

Test Plan:
Built with the maintainer mode build setting, in all three modes
(Release, Debug, and Valgrind).
Jenkins followed up with a full test set, all is clean.

Reviewers: santoshb

Reviewed By: santoshb
Add -std=c++11 to gcc builds

Summary:
We build with this, but pass it in from our build scripts.

This should be set on all C++ builds, so it should be done here.

Test Plan:
Built with it, with this option removed from our build tools.

Reviewers: santoshb

Reviewed By: santoshb