[findgstat] Use the dune cmake doc style. Add GSTAT_ROOT variable.

The user can set the variable GSTAT_ROOT to the directory where
gstat is installed for cmake to search there first before the
default paths.

cmake/modules/FindGstat.cmake

+# .. cmake_module::
 #
-# Module that checks whether gstat has been installed
+#    Find the gstat geostatistic library
+#
+#    You may set the following variables to modify the
+#    behaviour of this module:
+#
+#    :ref:`GSTAT_ROOT`
+#       Path list to search for gstat.
+#
+#    Sets the following variables:
+#
+#    :code:`GSTAT_FOUND`
+#       True if the gstat library was found.
+#
+#    :code:`GSTAT_EXECUTABLE`
+#       Path to gstat executable
+#
+# .. cmake_variable:: GSTAT_ROOT
+#
+#   You may set this variable to have :ref:`FindGstat` look
+#   for the gstat library in the given path before inspecting
+#   system paths.
 #
-# Sets the following variables:
-# GSTAT_FOUND         True if gstat was found
-# GSTAT_EXECUTABLE    Path to gstat executable
 
 # look for header files, only at positions given by the user
 find_program(GSTAT_EXECUTABLE
   NAMES gstat
-  PATHS "${CMAKE_SOURCE_DIR}/../external/gstat/src"
+  PATHS "${GSTAT_ROOT}"
+        "${CMAKE_SOURCE_DIR}/../"
         "usr/bin/"
+  PATH_SUFFIXES "src" "external/gstat/src" "gstat/src" "gstat"
+  NO_DEFAULT_PATH
 )
 
 include(FindPackageHandleStandardArgs)