Gazebo: improve handling of default GZ_VERSION

- Remove duplicate code and report error for invalid GZ_VERSION.

Signed-off-by: Rhys Mainwaring <rhys.mainwaring@me.com>

CMakeLists.txt

@@ -26,24 +26,17 @@ if("$ENV{GZ_VERSION}" STREQUAL "harmonic")
   set(GZ_SIM_VER ${gz-sim8_VERSION_MAJOR})
 
   message(STATUS "Compiling against Gazebo Harmonic")
-# Garden
-elseif("$ENV{GZ_VERSION}" STREQUAL "garden")
+# Garden (default)
+elseif("$ENV{GZ_VERSION}" STREQUAL "garden" OR NOT DEFINED "ENV{GZ_VERSION}")
   find_package(gz-cmake3 REQUIRED)
   set(GZ_CMAKE_VER ${gz-cmake3_VERSION_MAJOR})
 
   gz_find_package(gz-sim7 REQUIRED)
   set(GZ_SIM_VER ${gz-sim7_VERSION_MAJOR})
 
   message(STATUS "Compiling against Gazebo Garden")
-# Default to Garden
-else()
-  find_package(gz-cmake3 REQUIRED)
-  set(GZ_CMAKE_VER ${gz-cmake3_VERSION_MAJOR})
-
-  gz_find_package(gz-sim7 REQUIRED)
-  set(GZ_SIM_VER ${gz-sim7_VERSION_MAJOR})
-
-  message(STATUS "Compiling against Gazebo Garden")
+else()  
+  message(FATAL_ERROR "Unsupported GZ_VERSION: $ENV{GZ_VERSION}")
 endif()
 
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