add option to enable PETSc

disabled by default

cmake/Modules/FindPETSc.cmake

@@ -16,6 +16,11 @@
 
 # find out the size of a pointer. this is required to only search for
 # libraries in the directories relevant for the architecture
+
+if(NOT USE_PETSC)
+  return()
+endif()
+
 if (CMAKE_SIZEOF_VOID_P)
   math (EXPR _BITS "8 * ${CMAKE_SIZEOF_VOID_P}")
 endif (CMAKE_SIZEOF_VOID_P)

cmake/Modules/OpmLibMain.cmake

@@ -105,6 +105,9 @@ find_threads (${project})
 # SuperLU is optional
 option (USE_SUPERLU "Use SuperLU direct solvers" OFF)
 
+# PETSc is optional
+option (USE_PETSC "Use PETSc iterative solvers" OFF)
+
 # static code analysis
 include(UseStaticAnalysis)
 setup_static_analysis_tools()