Merge pull request #125 from nathanrossi/nathanrossi/fix-git-describe libtrellis/CMakeLists.txt: Run git describe in source directory

diff --git a/libtrellis/CMakeLists.txt b/libtrellis/CMakeLists.txt
index f51c44f..b19e727 100644
--- a/libtrellis/CMakeLists.txt
+++ b/libtrellis/CMakeLists.txt

@@ -129,7 +129,7 @@
 file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/generated")
 set(LAST_GIT_VERSION "")
 if (NOT DEFINED CURRENT_GIT_VERSION)
-    execute_process(COMMAND git describe --tags OUTPUT_VARIABLE CURRENT_GIT_VERSION)
+    execute_process(COMMAND git describe --tags OUTPUT_VARIABLE CURRENT_GIT_VERSION WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
 endif()
 string(STRIP "${CURRENT_GIT_VERSION}" CURRENT_GIT_VERSION)
 if (EXISTS "${CMAKE_BINARY_DIR}/generated/last_git_version")