Fix CMake macro special variable usage (#216)

The argument-related variables in a macro body are not real variables,
but special substitutions. They cannot be directly referred to by name,
only expanded.

cmake/utils.cmake

@@ -59,11 +59,13 @@ endmacro ()
 # variable. When gflags is a subproject of another project (GFLAGS_IS_SUBPROJECT),
 # the variable is not added to the CMake cache. Otherwise it is cached.
 macro (gflags_define type varname docstring default)
-  if (ARGC GREATER 5)
+  # note that ARGC must be expanded here, as it is not a "real" variable
+  # (see the CMake documentation for the macro command)
+  if ("${ARGC}" GREATER 5)
     message (FATAL_ERROR "gflags_variable: Too many macro arguments")
   endif ()
   if (NOT DEFINED GFLAGS_${varname})
-    if (GFLAGS_IS_SUBPROJECT AND ARGC EQUAL 5)
+    if (GFLAGS_IS_SUBPROJECT AND "${ARGC}" EQUAL 5)
       set (GFLAGS_${varname} "${ARGV4}")
     else ()
       set (GFLAGS_${varname} "${default}")
@@ -83,7 +85,9 @@ endmacro ()
 macro (gflags_property varname property value)
   gflags_is_cached (_cached ${varname})
   if (_cached)
-    if (property STREQUAL ADVANCED)
+    # note that property must be expanded here, as it is not a "real" variable
+    # (see the CMake documentation for the macro command)
+    if ("${property}" STREQUAL "ADVANCED")
       if (${value})
         mark_as_advanced (FORCE ${varname})
       else ()