Search for GMP if dune-common is used.

We do suggest dune-fem for opm-models and that one links to it
if available.

cmake/Modules/FindGMP.cmake

@@ -0,0 +1,31 @@
+# - Find the GNU Multiprecision library.
+#
+# Will define the following imported target for usage:
+# - GMP::gmp Target for linking/compiling with C library
+# - GMP::gmpxx Target for linking/compiling with C++ library
+find_path(GMP_INCLUDE_DIR gmp.h)
+find_library(GMP_LIBRARY gmp)
+find_path(GMPXX_INCLUDE_DIR gmpxx.h)
+find_library(GMPXX_LIBRARY gmpxx)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(GMP
+  DEFAULT_MSG
+  GMPXX_LIBRARY GMPXX_INCLUDE_DIR GMP_INCLUDE_DIR GMP_LIBRARY)
+
+if(GMP_FOUND)
+  if(NOT TARGET GMP::gmp)
+    add_library(GMP::gmp UNKNOWN IMPORTED GLOBAL)
+    set_target_properties(GMP::gmp PROPERTIES
+      IMPORTED_LOCATION ${GMP_LIBRARY}
+      INTERFACE_INCLUDE_DIRECTORIES ${GMP_INCLUDE_DIR})
+  endif()
+
+  if(NOT TARGET GMP::gmpxx)
+    add_library(GMP::gmpxx UNKNOWN IMPORTED GLOBAL)
+    set_target_properties(GMP::gmpxx PROPERTIES
+      IMPORTED_LOCATION ${GMPXX_LIBRARY}
+      INTERFACE_INCLUDE_DIRECTORIES ${GMPXX_INCLUDE_DIR}
+      TARGET_LINK_LIBRARIES GMP::gmp)
+  endif()
+endif()

cmake/Modules/Finddune-common.cmake

@@ -20,7 +20,8 @@ find_opm_package (
   LAPACK REQUIRED;
   CxaDemangle;
   MPI;
-  TBB
+  TBB;
+  GMP
   "
   # header to search for
   "dune/common/fvector.hh"