Add new function tribits_package_is_enabled_or_unset()

This helps to make the code more clear.

NOTE: This is tested as part of higher-level code and tests
(e.g. DependencyUnitTests tests).

tribits/core/package_arch/TribitsAdjustPackageEnables.cmake

@@ -945,7 +945,9 @@ macro(tribits_private_disable_required_package_enables
     fwdDepPkgName  packageName  libraryDep
   )
   tribits_get_package_enable_status(${fwdDepPkgName}  ""  fwdDepPkgEnableVarName)
-  if (${fwdDepPkgEnableVarName} OR "${${fwdDepPkgEnableVarName}}" STREQUAL "")
+  tribits_package_is_enabled_or_unset(${fwdDepPkgEnableVarName}
+    fwdDepPkgIsEnabledOrUnset)
+  if (fwdDepPkgIsEnabledOrUnset)
     if ("${libraryDep}" STREQUAL "TRUE")
       tribits_private_print_disable_required_package_enable(
         ${packageName}  ${fwdDepPkgEnableVarName}

tribits/core/package_arch/TribitsGetPackageEnableStatus.cmake

@@ -61,6 +61,34 @@ function(tribits_get_package_enable_status  packageName  packageEnableOut
 endfunction()
 
 
+# @FUNCTION: tribits_package_is_enabled_or_unset()
+#
+# Function that determines if a package's enable variable evaluates to true or
+# is unset.
+#
+# Usage::
+#
+#   tribits_package_is_enabled_or_unset((<packageEnableVarName>
+#     <packageIsEnabledOrUnsetOut>)
+#
+# On return, the value of ``<packageIsEnabledOrUnsetOut>`` will set to
+# ``TRUE`` if the variable ``<packageEnableVarName>`` evaluates to true and
+# or is empty "".  Otherwise, ``<packageIsEnabledOrUnsetOut>`` will set
+# to ``FALSE`` on return.
+#
+function(tribits_package_is_enabled_or_unset  packageEnableVarName
+    packageIsEnabledOrUnsetOut
+  )
+  if (${packageEnableVarName} OR ("${${packageEnableVarName}}" STREQUAL ""))
+    set(packageIsEnabledOrUnset TRUE)
+  else()
+    set(packageIsEnabledOrUnset FALSE)
+  endif()
+  set(${packageIsEnabledOrUnsetOut} ${packageIsEnabledOrUnset}
+    PARENT_SCOPE)
+endfunction()
+
+
 # @FUNCTION: tribits_package_is_explicitly_disabled()
 #
 # Function that determines if a package's enable variable is

tribits/doc/guides/TribitsMacroFunctionDocTemplate.rst

@@ -40,6 +40,7 @@
 @MACRO:    tribits_package_def() +
 @MACRO:    tribits_package_define_dependencies() +
 @MACRO:    tribits_process_enabled_standard_tpl() +
+@FUNCTION: tribits_package_is_enabled_or_unset() +
 @FUNCTION: tribits_package_is_explicitly_disabled() +
 @MACRO:    tribits_package_postprocess() +
 @MACRO:    tribits_process_subpackages() +