[dev] add -D_POSIX_C_SOURCE=200809L for metis build (dmlc#7565)

Author: Rhett-Ying

CMakeLists.txt

@@ -373,6 +373,12 @@ else(EXTERNAL_METIS_PATH)
   target_include_directories(dgl PRIVATE "third_party/METIS/include")
   # Compile METIS
   if(NOT MSVC)
+    # When building on ubi7, it fails with the following error:
+    # /usr/include/signal.h:156:29: error: unknown type name 'siginfo_t'.
+    # So I(Rui) added the following 2 lines to fix the issue.
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_POSIX_C_SOURCE=200809L")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809L")
+
     set(GKLIB_PATH "${CMAKE_CURRENT_SOURCE_DIR}/third_party/GKlib")
     include(${GKLIB_PATH}/GKlibSystem.cmake)
     include_directories(${GKLIB_PATH})