Don't run hg on non mercurial clone

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5245b29..91545bd 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,10 +41,13 @@
 set(EIGEN_MINOR_VERSION "${CMAKE_MATCH_1}")
 set(EIGEN_VERSION_NUMBER ${EIGEN_WORLD_VERSION}.${EIGEN_MAJOR_VERSION}.${EIGEN_MINOR_VERSION})
 
-# if the mercurial program is absent, this will leave the EIGEN_HG_CHANGESET string empty,
-# but won't stop CMake.
-execute_process(COMMAND hg tip -R ${CMAKE_SOURCE_DIR} OUTPUT_VARIABLE EIGEN_HGTIP_OUTPUT)
-execute_process(COMMAND hg branch -R ${CMAKE_SOURCE_DIR} OUTPUT_VARIABLE EIGEN_BRANCH_OUTPUT)
+# if we are not in a mercurial clone
+if(IS_DIRECTORY ${CMAKE_SOURCE_DIR}/.hg)
+  # if the mercurial program is absent or this will leave the EIGEN_HG_CHANGESET string empty,
+  # but won't stop CMake.
+  execute_process(COMMAND hg tip -R ${CMAKE_SOURCE_DIR} OUTPUT_VARIABLE EIGEN_HGTIP_OUTPUT)
+  execute_process(COMMAND hg branch -R ${CMAKE_SOURCE_DIR} OUTPUT_VARIABLE EIGEN_BRANCH_OUTPUT)
+endif()
 
 # if this is the default (aka development) branch, extract the mercurial changeset number from the hg tip output...
 if(EIGEN_BRANCH_OUTPUT MATCHES "default")