Add utility macro to conditionally print CMake variables

When debugging configuration issues, I often find that it would be helpful to print out a subset of CMake's variables and their values.

I'd like to add a new utility macro that can filter the variables and their values by a regular expression and then print the results.
E.g. something like:
```cmake
blt_print_variables(
[NAME_REGEX ] # optional
[VALUE_REGEX ] # optional
[ENV TRUE|FALSE] # optional, include environment variables in results, default is false
)
```

Here is a simple script to print out all values (without filtering):
```cmake
get_cmake_property(_variableNames VARIABLES)
list (SORT _variableNames)
foreach (_variableName ${_variableNames})
message(STATUS "${_variableName}=${${_variableName}}")
endforeach()
```
from: https://stackoverflow.com/a/9328525

Start from the proposed blt_print_variables(...) entry point and compare it with the provided get_cmake_property/list/foreach example. Define the behavior for optional NAME_REGEX, VALUE_REGEX, and ENV filters, then verify that matching variable names and values are printed in sorted order. Done means the macro works for filtered and unfiltered cases and has coverage in the project’s existing test setup.

Written by the indexing model from the issue text.