Use `EXCLUDE_FROM_ALL` in `add_subdirectory` to prevent `make install` from including lots of headers from gtest/gmock.
```
add_subdirectory(${CMAKE_BINARY_DIR}/googletest-src
${CMAKE_BINARY_DIR}/googletest-build
EXCLUDE_FROM_ALL)
```
Adds instructions for how to add gtest and gmock to
another CMake project directly. Downloading of the
googletest sources happens as configure time, allowing
it to be added to the main build directly via the
add_subdirectory() command. This ensures googletest is
built with the same compiler settings, etc. and will
typically result in a more robust and more convenient
build arrangement.
Put occurrences of "#include" in a code span so they are not interpreted
as headers.
Other documents were not broken because the #include was not at the
start of the line, but put them in code spans anyway just in case the
text gets refilled in the future.
