BUG: Be more careful with Boost_MINOR_VERSION in FindBoost module

Modules/FindBoost.cmake

@@ -376,15 +376,21 @@ ELSE (_boost_IN_CACHE)
       STRING(REGEX REPLACE "([0-9])\\.([0-9])\\.[0-9]" "\\1\\2"
         _boost_COMPILER_VERSION ${_boost_COMPILER_VERSION})
       IF(APPLE)
-        IF (${Boost_MINOR_VERSION} GREATER 35)
-          # In Boost 1.36.0 and newer, the mangled compiler name used
-          # on Mac OS X/Darwin is "xgcc".
-          SET (_boost_COMPILER "-xgcc${_boost_COMPILER_VERSION}")
-        ELSE()
-          # In Boost <= 1.35.0, there is no mangled compiler name for
-          # the Mac OS X/Darwin version of GCC.
-          SET (_boost_COMPILER "")
-        ENDIF()
+        IF(Boost_MINOR_VERSION)
+          IF(${Boost_MINOR_VERSION} GREATER 35)
+            # In Boost 1.36.0 and newer, the mangled compiler name used
+            # on Mac OS X/Darwin is "xgcc".
+            SET(_boost_COMPILER "-xgcc${_boost_COMPILER_VERSION}")
+          ELSE(${Boost_MINOR_VERSION} GREATER 35)
+            # In Boost <= 1.35.0, there is no mangled compiler name for
+            # the Mac OS X/Darwin version of GCC.
+            SET(_boost_COMPILER "")
+          ENDIF(${Boost_MINOR_VERSION} GREATER 35)
+        ELSE(Boost_MINOR_VERSION)
+          # We don't know the Boost version, so assume it's
+          # pre-1.36.0.
+          SET(_boost_COMPILER "")
+        ENDIF(Boost_MINOR_VERSION)
       ELSE()
         SET (_boost_COMPILER "-gcc${_boost_COMPILER_VERSION}")
       ENDIF()