Optionalized build dependency on git

Signed-off-by: Bill Morris <bill@sevendof.com>

CMakeLists.txt

@@ -47,12 +47,14 @@ message(STATUS "build type is ${CMAKE_BUILD_TYPE}")
 
 set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS "Debug;Release;RelWithDebInfo;MinSizeRel;Coverage")
 
-execute_process(
-	COMMAND git describe --always --tags
-	OUTPUT_VARIABLE git_tag
-	OUTPUT_STRIP_TRAILING_WHITESPACE
-	WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-	)
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/.git)
+	execute_process(
+		COMMAND git describe --always --tags
+		OUTPUT_VARIABLE git_tag
+		OUTPUT_STRIP_TRAILING_WHITESPACE
+		WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+		)
+endif()
 
 message(STATUS "PX4 ECL: Very lightweight Estimation & Control Library ${git_tag}")