PERF: Remove OpenMP specific CMake code

- Following pull request SuperElastix/elastix#1188
"Remove OpenMP based code, ELASTIX_USE_OPENMP option and OpenMP compiler flags"

CMakeLists.txt

@@ -28,11 +28,6 @@ if(NOT DEFINED USE_KNNGraphAlphaMutualInformationMetric)
   set(USE_KNNGraphAlphaMutualInformationMetric OFF CACHE BOOL "Use KNN metric.  Requires ANN library.")
 endif()
 
-if(CMAKE_COMPILER_IS_GNUCXX AND
-  ("${CMAKE_CXX_COMPILER_VERSION}" VERSION_EQUAL "4.8") OR
-  ("${CMAKE_CXX_COMPILER_VERSION}" VERSION_GREATER "4.8" AND "${CMAKE_CXX_COMPILER_VERSION}" VERSION_LESS "5.0") )
-  set(ELASTIX_USE_OPENMP OFF CACHE BOOL "Use OpenMP in elastix")
-endif()
 if(CMAKE_COMPILER_IS_GNUCXX)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-aggressive-loop-optimizations")
 endif()
@@ -61,7 +56,6 @@ if(SKBUILD)
 endif()
 
 if(WASI OR EMSCRIPTEN)
-  option(ELASTIX_USE_OPENMP "OpenMP not supported on WASI or Emscripten" OFF)
   option(ELASTIX_NO_INSTALL_EXECUTABLES "Do not install executables" ON)
 endif()
 
@@ -91,17 +85,6 @@ find_package(Elastix REQUIRED)
 include_directories( ${ELASTIX_INCLUDE_DIRS} )
 link_directories( ${ELASTIX_LIBRARY_DIRS} )
 #include(${ELASTIX_USE_FILE})
-if(ELASTIX_USE_OPENMP)
-  find_package(OpenMP QUIET)
-  if(OPENMP_FOUND)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
-    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${OpenMP_EXE_LINKER_FLAGS}")
-    set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${OpenMP_SHARED_LINKER_FLAGS}")
-    set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} ${OpenMP_STATIC_LINKER_FLAGS}")
-    add_definitions(-DELASTIX_USE_OPENMP)
-  endif()
-endif()
 if(ELASTIX_USE_OPENCL)
   add_definitions(-DELASTIX_USE_OPENCL)
 endif()