CMake: avoid invoking Git in Dev builds

Avoid running FindGit and git process on every build, when in Developer
mode: this feature is only useful for CI builds, and remove it makes
the edit-and-debug cycle slightly faster. This means buildId.h contains
dummy values when FG_BUILD_TYPE=Dev

CMakeModules/GenerateBuildInfo.cmake

@@ -1,8 +1,24 @@
 
-add_custom_target(
-  buildId
-  ${CMAKE_COMMAND} -D SRC=${CMAKE_SOURCE_DIR}
-                   -D DST=${CMAKE_BINARY_DIR}/src/Include/flightgearBuildId.h
-                   -P ${CMAKE_SOURCE_DIR}/CMakeModules/buildId.cmake
-)
+set(BUILD_ID_DST_PATH "${CMAKE_BINARY_DIR}/src/Include/flightgearBuildId.h")
 
+if (FG_BUILD_TYPE STREQUAL "Dev")
+  # default values for all of these
+  set(JENKINS_BUILD_ID "none")
+  set(JENKINS_BUILD_NUMBER 0)
+  set(REVISION "none")
+
+  # generate a placeholder buildId header
+  configure_file (${CMAKE_SOURCE_DIR}/src/Include/flightgearBuildId.h.cmake-in ${BUILD_ID_DST_PATH})
+
+  # dummy target, does nothing
+  add_custom_target(buildId)
+else()
+  # for non-Dev builds, run this each time. using some CMake in script mode
+  # this takes a little bit of time since we have to actually run Git as a sub-process
+  add_custom_target(
+    buildId
+    ${CMAKE_COMMAND} -D SRC=${CMAKE_SOURCE_DIR}
+                    -D DST=${BUILD_ID_DST_PATH}
+                    -P ${CMAKE_SOURCE_DIR}/CMakeModules/buildId.cmake
+  )
+endif()