Draft: adding cpp version of codes

What is the new feature

Creating the C++ version of existing code

The people involved in porting the existing codes to C++ are @b382190, @b382899, @m301159, @k202204, @k202194, @k202174 and @k202170

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)
• Prior to merging, please remove any boilerplate from the MR description, retaining only the Please describe your feature in a couple of words and describe important implementation details of the feature section to maintain

You are not supposed to merge this request by yourself, the maintainers of libiconmath take care of this action!

src/horizontal/CMakeLists.txt

@@ -11,6 +11,7 @@
 
 add_library(
   iconmath-horizontal
+  mo_lib_divrot.cpp
   mo_lib_divrot.F90
   mo_lib_laplace.F90
   mo_lib_gradients.F90)
@@ -39,7 +40,9 @@ if(IM_ENABLE_OPENACC)
   # If _OPENACC is defined, assume that the required compiler flags are already
   # provided, e.g. in CMAKE_Fortran_FLAGS:
   if(NOT HAS_OPENACC_MACRO)
-    target_compile_options(iconmath-horizontal PRIVATE ${OpenACC_Fortran_OPTIONS})
+    target_compile_options(iconmath-horizontal
+                           PRIVATE
+                           $<$<COMPILE_LANGUAGE:Fortran>:${OpenACC_Fortran_OPTIONS}>)
     # This make sures that unit tests (FortUTF) compiles without the need of
     # passing OpenACC compile option.
     target_link_libraries(iconmath-horizontal PRIVATE OpenACC::OpenACC_Fortran)
@@ -52,11 +55,22 @@ target_include_directories(
     # Path to the Fortran modules:
     $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:Fortran>:${Fortran_MODULE_DIRECTORY}>>
     $<INSTALL_INTERFACE:$<$<COMPILE_LANGUAGE:Fortran>:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>>
+    # Path to the internal C/C++ headers (for testing): Requires CMake 3.15+ for
+    # multiple compile languages
+    # https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html
+    $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C,CXX>:${PROJECT_SOURCE_DIR}/src>>
+  PRIVATE
+    # Path to config.h (for C and C++ only): Requires CMake 3.15+ for multiple
+    # compile languages
+    # https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html
+    $<BUILD_INTERFACE:$<$<COMPILE_LANGUAGE:C,CXX>:${CMAKE_CURRENT_BINARY_DIR}>>
 )
 
 target_link_libraries(iconmath-horizontal PUBLIC fortran-support::fortran-support)
 target_link_libraries(iconmath-horizontal PUBLIC iconmath-support)
 target_link_libraries(iconmath-horizontal PUBLIC iconmath-interpolation)
+target_link_libraries(iconmath-horizontal PRIVATE Kokkos::kokkos)
+set_target_properties(iconmath-horizontal PROPERTIES LINKER_LANGUAGE Fortran)
 
 install(TARGETS iconmath-horizontal EXPORT "${PROJECT_NAME}-targets")