Support building against system libraries

Problem
-------

**Given**:

A sandbox environment is an environment where dependencies
are all provided but access to the internet is not. Also take as
a given that the build for this project happens in an environment
that meets that description.

**When**:

Configuring this project with any of the provided approaches. For
instance:

```
cmake -B build -S .
```

**Then**:

The configuration step hangs or fails, depending on the nature of
the sandboxed environment.

Solution
--------

Introduce a `BEMAN_EXEMPLAR_FETCH_GOOGLETEST` option to allow the
user to disable actions to download files from the internet without
disabling the building and running of tests. Those dependencies will
be provided correctly such that `find_package` calls will complete
successfully.

This workflow will provide minimal support for all known existing
packaging approaches, though `BEMAN_EXEMPLAR_FETCH_GOOGLETEST`
option will need to be explicitly set to a falsey value.

Future commits can revisit how to balance `FetchContent` and
`find_package` approaches to configuring CMake projects.

CMakeLists.txt

@@ -11,36 +11,44 @@ project(
 
 enable_testing()
 
-# [CMAKE.SKIP_TESTS]
 option(
     BEMAN_EXEMPLAR_BUILD_TESTS
-    "Enable building tests and test infrastructure. Default: ON. Values: { ON, OFF }."
+    "Enable building tests and test infrastructure."
     ${PROJECT_IS_TOP_LEVEL}
 )
 
-# [CMAKE.SKIP_EXAMPLES]
 option(
     BEMAN_EXEMPLAR_BUILD_EXAMPLES
-    "Enable building examples. Default: ON. Values: { ON, OFF }."
+    "Enable building examples."
+    ${PROJECT_IS_TOP_LEVEL}
+)
+
+option(
+    BEMAN_EXEMPLAR_FETCH_GOOGLETEST
+    "Enable downloading and building googletest from source."
     ${PROJECT_IS_TOP_LEVEL}
 )
 
 include(FetchContent)
 include(GNUInstallDirs)
 
 if(BEMAN_EXEMPLAR_BUILD_TESTS)
-    # Fetch GoogleTest
-    FetchContent_Declare(
-        googletest
-        GIT_REPOSITORY https://github.com/google/googletest.git
-        GIT_TAG
-            f8d7d77c06936315286eb55f8de22cd23c188571 # release-1.14.0
-        EXCLUDE_FROM_ALL
-    )
-    block()
-        set(INSTALL_GTEST OFF) # Disable GoogleTest installation
-        FetchContent_MakeAvailable(googletest)
-    endblock()
+    if(BEMAN_EXEMPLAR_FETCH_GOOGLETEST)
+        # Fetch GoogleTest
+        FetchContent_Declare(
+            googletest
+            GIT_REPOSITORY https://github.com/google/googletest.git
+            GIT_TAG
+                f8d7d77c06936315286eb55f8de22cd23c188571 # release-1.14.0
+            EXCLUDE_FROM_ALL
+        )
+        block()
+            set(INSTALL_GTEST OFF) # Disable GoogleTest installation
+            FetchContent_MakeAvailable(googletest)
+        endblock()
+    else()
+        find_package(GTest REQUIRED)
+    endif()
 endif()
 
 add_subdirectory(src/beman/exemplar)