[cmake] Only require git when running benchmarks.

This should improve the current infrastructure for releasing clad on conda.
The conda recipe downloads the tarball and it currently fails because we rely
on being inside a git setup when building.

cmake/modules/AddClad.cmake

@@ -1,15 +1,18 @@
-# Find the current branch.
-execute_process(WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-                COMMAND git rev-parse HEAD
-                OUTPUT_VARIABLE CURRENT_REPO_COMMIT
-                OUTPUT_STRIP_TRAILING_WHITESPACE)
-string(REPLACE "/" "" CURRENT_REPO_COMMIT ${CURRENT_REPO_COMMIT})
+if (CLAD_ENABLE_BENCHMARKS)
+  # Find the current branch.
+  execute_process(WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+                  COMMAND git rev-parse HEAD
+                  OUTPUT_VARIABLE CURRENT_REPO_COMMIT
+                  OUTPUT_STRIP_TRAILING_WHITESPACE)
+  string(REPLACE "/" "" CURRENT_REPO_COMMIT ${CURRENT_REPO_COMMIT})
 
 # Ask cmake to reconfigure each time we change the branch so that it can change
 # the value of CURRENT_REPO_COMMIT.
 set_property(DIRECTORY APPEND PROPERTY
              CMAKE_CONFIGURE_DEPENDS "${CMAKE_SOURCE_DIR}/.git/HEAD")
 
+endif(CLAD_ENABLE_BENCHMARKS)
+
 #-------------------------------------------------------------------------------
 # function ENABLE_CLAD_FOR_EXECUTABLE(<executable>
 #   DEPENDS dependencies...