--- ThirdParty/hdf5/vtkhdf5/CMakeLists.txt.org	2020-08-17 15:13:30.938786723 +0200
+++ ThirdParty/hdf5/vtkhdf5/CMakeLists.txt	2020-08-17 15:14:07.401037024 +0200
@@ -412,14 +412,14 @@
       # timezone is not included in time.h on mingw if -std=c99
       set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -finline-functions -fno-common")
     else ()
-      set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -std=c99 -finline-functions -fno-common")
+      set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -std=gnu99 -finline-functions -fno-common")
     endif()
   else (CMAKE_BUILD_TYPE MATCHES Debug)
     if (WIN32)
       # timezone is not included in time.h on mingw if -std=c99
       set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -fomit-frame-pointer -finline-functions -fno-common")
     else ()
-      set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -std=c99 -fomit-frame-pointer -finline-functions -fno-common")
+      set (CMAKE_C_FLAGS "${CMAKE_ANSI_CFLAGS} ${CMAKE_C_FLAGS} -std=gnu99 -fomit-frame-pointer -finline-functions -fno-common")
     endif ()
   endif (CMAKE_BUILD_TYPE MATCHES Debug)
 endif (CMAKE_COMPILER_IS_GNUCC)
