Various compilation fixes

Various compilation errors I encountered on Windows, macOS and Linux.

OpenMP is now handled in OTB itself with the OTB_USE_OPENMP setting.

CMakeLists.txt

@@ -5,13 +5,17 @@ if(NOT OTB_SOURCE_DIR)
     find_package(OTB REQUIRED)
     list(APPEND CMAKE_MODULE_PATH ${OTB_CMAKE_DIR})
     include(${OTB_USE_FILE})
+    if(NOT OTB_USE_OPENMP)
+      find_package(OpenMP REQUIRED)
+      set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
+    endif()
     include(OTBModuleExternal)
 else()
     otb_module_impl()
-endif()
-
-if(NOT OTB_USE_OPENMP)
-  message(FATAL_ERROR "Diapotb remote module needs OpenMP, compile OTB with OTB_USE_OPENMP=ON")
+    if(NOT OTB_USE_OPENMP)
+      message(FATAL_ERROR "Diapotb remote module needs OpenMP, compile OTB with OTB_USE_OPENMP=ON")
+    endif()
 endif()
 
 # Execute and Copy during installation