[FR]: Make default mock behavior an option

### Does the feature exist in the most recent commit?

No.

### Why do we need this feature?

I would like to have a default `NiceMock` behavior of the mock classes.

The default mock behavior that is set in `gmock.cc` is currently hard-coded, so it can only be changed by modifying the source. A comment in `gmock-nice-strict.h` states that "we will soon switch the default behavior of mocks to be nice". However, that comment is more than 13 years old, so I assume 'soon' will be 'never'. And I doubt that such a change would be accepted, because the default is a user preference.

### Describe the proposal.

My proposal is to make the default mock behavior a CMake option, so every user can choose the behavior without modifying the original source code.

Simple example to get an idea:
`CMakeLists.txt`:
```
set(DEFAULT_MOCK_BEHAVIOR "1" CACHE STRING "Default mock behavior.")
target_compile_definitions(gmock PUBLIC DEFAULT_MOCK_BEHAVIOR=${DEFAULT_MOCK_BEHAVIOR})
```
`gmock.cc`:
```
GMOCK_DEFINE_int32_(default_mock_behavior, DEFAULT_MOCK_BEHAVIOR,
```

### Is the feature specific to an operating system, compiler, or build system version?

No.

Start by inspecting CMakeLists.txt and gmock.cc, then read the default-behavior context in gmock-nice-strict.h. Trace how the current hard-coded default is passed into the mock configuration. Done means users can select the default mock behavior through a CMake option without modifying the source.

Written by the indexing model from the issue text.