enable c99 if available (for hxt)

CMakeLists.txt

@@ -45,6 +45,7 @@ opt(CGNS "Enable CGNS mesh export (experimental)" OFF)
 opt(CAIRO "Enable Cairo to render fonts (experimental)" ${DEFAULT})
 opt(COMPRESSED_IO "Enable compressed (gzip) input/output using zlib" OFF)
 opt(CXX11 "Enable C++11" ${DEFAULT})
+opt(C99 "Enable C99" ${DEFAULT})
 opt(DINTEGRATION "Enable discrete integration (needed for levelsets)" ${DEFAULT})
 opt(FLTK "Enable FLTK graphical user interface (requires mesh/post)" ${DEFAULT})
 opt(FOURIER_MODEL "Enable Fourier geometrical models (experimental)" OFF)
@@ -166,6 +167,7 @@ include(CheckTypeSize)
 include(CheckFunctionExists)
 include(CheckIncludeFile)
 include(CheckCXXCompilerFlag)
+include(CheckCCompilerFlag)
 
 macro(set_config_option VARNAME STRING)
   set(${VARNAME} TRUE)
@@ -236,6 +238,14 @@ if(ENABLE_CXX11)
   endif(STDCXX11)
 endif(ENABLE_CXX11)
 
+if(ENABLE_C99)
+  # in recent cmake versions we could do e.g. set(CMAKE_C_STANDARD 99)
+  check_c_compiler_flag("-std=c99" STDC99)
+  if(STDC99)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
+  endif(STDC99)
+endif(ENABLE_C99)
+
 macro(append_gmsh_src DIRNAME FILES)
   foreach(FILE ${FILES})
     list(APPEND LIST ${DIRNAME}/${FILE})