Be more robust if neither .git nor .tag file can not be found.

Before, cmake was bailing out. Fixes #211

CMakeLists.txt

@@ -25,11 +25,15 @@ get_git_head_revision(GIT_REFSPEC GIT_SHA1)
 # this will work if the tar balls have been properly created
 # via git-archive.
 if ("${GIT_SHA1}" STREQUAL "GITDIR-NOTFOUND")
-    file(READ ${CMAKE_SOURCE_DIR}/.tag sha1_candidate)
-    string(REPLACE "\n" "" sha1_candidate ${sha1_candidate})
-    if (NOT ${sha1_candidate} STREQUAL "$Format:%H$")
-        message("${sha1_candidate}")
-        set (GIT_SHA1 "${sha1_candidate}")
+    if (EXISTS "${CMAKE_SOURCE_DIR}/.tag")
+        file(READ ${CMAKE_SOURCE_DIR}/.tag sha1_candidate)
+        string(REPLACE "\n" "" sha1_candidate ${sha1_candidate})
+        if (NOT ${sha1_candidate} STREQUAL "$Format:%H$")
+            message("${sha1_candidate}")
+            set (GIT_SHA1 "${sha1_candidate}")
+        endif()
+    else()
+        set (GIT_SHA1 "unknown")
     endif()
 endif()