3.14 is used and tested by linux mini-product
to fix
```
CMake Error at CMakeLists.txt:4 (cmake_minimum_required):
Compatibility with CMake < 3.5 has been removed from CMake.
Update the VERSION argument <min> value. Or, use the <min>...<max> syntax
to tell CMake that the project requires at least <min> but has been updated
to work with policies introduced by <max> or earlier.
Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
```
Use the cmake variable `WAMR_BUILD_GLOBAL_HEAP_POOL` and
`WAMR_BUILD_GLOBAL_HEAP_SIZE` to enable/disable the global heap pool
and set its size. And set the default global heap size in core/config.h and
the cmake files.
As a result, the developers who build iwasm can easily enable/disable the
global heap pool and change its size regardless of the iwasm implementation,
without manually finding and patching the right location for that value.
