FindRuby: do not blindly set version to 1.8.0

RUBY_VERSION was always set, even if no RUBY_EXECUTABLE was found. While it
may make sense to assume a default version if we can't execute the binary, it
certainly doesn't make sense to report a version if there is no executable at
all.

Modules/FindRuby.cmake

@@ -139,7 +139,7 @@ ENDIF(RUBY_EXECUTABLE  AND NOT  RUBY_VERSION_MAJOR)
 
 # In case RUBY_EXECUTABLE could not be executed (e.g. cross compiling)
 # try to detect which version we found. This is not too good.
-IF(NOT RUBY_VERSION_MAJOR)
+IF(RUBY_EXECUTABLE AND NOT RUBY_VERSION_MAJOR)
    # by default assume 1.8.0
    SET(RUBY_VERSION_MAJOR 1)
    SET(RUBY_VERSION_MINOR 8)
@@ -149,13 +149,14 @@ IF(NOT RUBY_VERSION_MAJOR)
       SET(RUBY_VERSION_MAJOR 1)
       SET(RUBY_VERSION_MINOR 9)
    ENDIF(${RUBY_EXECUTABLE} MATCHES "ruby1.?9"  OR  RUBY_HDR_DIR)
-ENDIF(NOT RUBY_VERSION_MAJOR)
+ENDIF(RUBY_EXECUTABLE AND NOT RUBY_VERSION_MAJOR)
 
-
-SET(RUBY_VERSION "${RUBY_VERSION_MAJOR}.${RUBY_VERSION_MINOR}.${RUBY_VERSION_PATCH}")
-SET(_RUBY_VERSION_SHORT "${RUBY_VERSION_MAJOR}.${RUBY_VERSION_MINOR}")
-SET(_RUBY_VERSION_SHORT_NODOT "${RUBY_VERSION_MAJOR}${RUBY_VERSION_MINOR}")
-SET(_RUBY_NODOT_VERSION "${RUBY_VERSION_MAJOR}${RUBY_VERSION_MINOR}${RUBY_VERSION_PATCH}")
+IF(RUBY_VERSION_MAJOR)
+   SET(RUBY_VERSION "${RUBY_VERSION_MAJOR}.${RUBY_VERSION_MINOR}.${RUBY_VERSION_PATCH}")
+   SET(_RUBY_VERSION_SHORT "${RUBY_VERSION_MAJOR}.${RUBY_VERSION_MINOR}")
+   SET(_RUBY_VERSION_SHORT_NODOT "${RUBY_VERSION_MAJOR}${RUBY_VERSION_MINOR}")
+   SET(_RUBY_NODOT_VERSION "${RUBY_VERSION_MAJOR}${RUBY_VERSION_MINOR}${RUBY_VERSION_PATCH}")
+ENDIF(RUBY_VERSION_MAJOR)
 
 FIND_PATH(RUBY_INCLUDE_DIR
    NAMES ruby.h
@@ -167,7 +168,7 @@ FIND_PATH(RUBY_INCLUDE_DIR
 SET(RUBY_INCLUDE_DIRS ${RUBY_INCLUDE_DIR} )
 
 # if ruby > 1.8 is required or if ruby > 1.8 was found, search for the config.h dir
-IF( ${Ruby_FIND_VERSION_SHORT_NODOT} GREATER 18  OR  ${_RUBY_VERSION_SHORT_NODOT} GREATER 18  OR  RUBY_HDR_DIR)
+IF( "${Ruby_FIND_VERSION_SHORT_NODOT}" GREATER 18  OR  "${_RUBY_VERSION_SHORT_NODOT}" GREATER 18  OR  RUBY_HDR_DIR)
    FIND_PATH(RUBY_CONFIG_INCLUDE_DIR
      NAMES ruby/config.h  config.h
      HINTS
@@ -176,7 +177,7 @@ IF( ${Ruby_FIND_VERSION_SHORT_NODOT} GREATER 18  OR  ${_RUBY_VERSION_SHORT_NODOT
      )
 
    SET(RUBY_INCLUDE_DIRS ${RUBY_INCLUDE_DIRS} ${RUBY_CONFIG_INCLUDE_DIR} )
-ENDIF( ${Ruby_FIND_VERSION_SHORT_NODOT} GREATER 18  OR  ${_RUBY_VERSION_SHORT_NODOT} GREATER 18  OR  RUBY_HDR_DIR)
+ENDIF( "${Ruby_FIND_VERSION_SHORT_NODOT}" GREATER 18  OR  "${_RUBY_VERSION_SHORT_NODOT}" GREATER 18  OR  RUBY_HDR_DIR)
 
 
 # Determine the list of possible names for the ruby library