Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Major improvement to build parallelism for googletest internal tests

Currently the googletest internal tests build after the matching library.
However, each of these is serialized at the top level makefile.
Additionally some of the tests (e.g. the gmock-matches-test) take up to
90 seconds to build with clang -O2. Having to wait for this test to
complete before continuing to the next directory seriously slows down the
parllelism of a -j32 build.
Before this change running `make -C lib/googletest -j32 -s` in buildenv
took 202 seconds, now it's 153 due to improved parallelism.

Reviewed By: emaste (no objection)
Differential Revision: https://reviews.freebsd.org/D26748

Sprinkle `${HAS_TESTS}` in Makefiles

While here, reindent `SUBDIR.${MK_TESTS}`

Fix up dependency finding for libg{mock,test}_main

- Look up the corresponding non-*_main libraries via LDFLAGS using the
directories provided in src.libnames.mk. This will allow the libraries to
be built in the "make libraries" phase of buildworld.
- gtest_main relies on gtest, but didn't explicitly call out the dependency
in `LIBADD`. Fill in this missing blank.

Merge build glue for libraries and tests done on github

I need to doublecheck my work vs the port, but I believe that this covers the
initial integration of all upstream tests.

Ref: https://github.com/ngie-eign/freebsd/tree/googletest-integration