Feature/petsc cmake

added cmake modules FindPETSc

cmake/modules/AmdisMacros.cmake

 include(AmdisCXXFeatures)
 include(AddAmdisExecutable)
 
-set(BACKEND "ISTL" CACHE STRING "LinearAlgebra backend. One of MTL, EIGEN, ISTL")
-set_property(CACHE BACKEND PROPERTY STRINGS "MTL" "EIGEN" "ISTL")
+if (NOT BACKEND)
+  set(BACKEND "ISTL" CACHE STRING "LinearAlgebra backend. One of MTL, EIGEN, PETSC, ISTL")
+  set_property(CACHE BACKEND PROPERTY STRINGS "MTL" "EIGEN" "ISTL" "PETSC")
+endif (NOT BACKEND)
 
 if (BACKEND STREQUAL "MTL" OR BACKEND STREQUAL "MTL4")
   find_package(MTL REQUIRED)
-  set(HAVE_MTL MTL_FOUND)
+  set(HAVE_MTL TRUE)
   if (MTL_FOUND)
     message(STATUS "  Found MTL, version: ${MTL_VERSION}")
     dune_register_package_flags(LIBRARIES MTL::MTL COMPILE_DEFINITIONS "ENABLE_MTL=1")
   endif (MTL_FOUND)
 elseif (BACKEND STREQUAL "EIGEN" OR BACKEND STREQUAL "EIGEN3")
   find_package(Eigen3 REQUIRED 3.3.5)
-  set(HAVE_EIGEN EIGEN_FOUND)
+  set(HAVE_EIGEN TRUE)
   if (EIGEN3_FOUND)
     message(STATUS "  Found Eigen3, version: ${Eigen3_VERSION}")
     dune_register_package_flags(LIBRARIES Eigen3::Eigen COMPILE_DEFINITIONS "ENABLE_EIGEN=1")
   endif (EIGEN3_FOUND)
+elseif (BACKEND STREQUAL "PETSC")
+  find_package(PETSc REQUIRED)
+  if (PETSc_FOUND)
+    set(HAVE_PETSC TRUE)
+    dune_register_package_flags(LIBRARIES PETSc::PETSc COMPILE_DEFINITIONS "ENABLE_PETSC=1")
+  endif (PETSc_FOUND)
 elseif (BACKEND STREQUAL "ISTL")
   if (NOT dune-istl_FOUND)
-    message(FATAL_ERROR "Need dune-istl, MTL, or Eigen3 as linear algebra backend. Change flag BACKEND!")
+    message(FATAL_ERROR "Need dune-istl, MTL, PETSc, or Eigen3 as linear algebra backend. Change flag BACKEND!")
   endif ()
 else ()
   message(FATAL_ERROR "BACKEND must be one of MTL, EIGEN, ISTL")
 endif ()
+
+
+set(DUNE_CUSTOM_PKG_CONFIG_SECTION "set(BACKEND \"${BACKEND}\")")
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