espeak-ng requires a C11 compiler, so document and check this

README.md

 
 1.  a functional autotools system (`make`, `autoconf`, `automake`, `libtool`
     and `pkg-config`);
-2.  a functional c++ compiler;
-3.  wxWidgets development libraries (needed to build and run espeakedit
+2.  a functional c compiler that supports C11;
+3.  a functional c++ compiler;
+4.  wxWidgets development libraries (needed to build and run espeakedit
     to compile the phoneme data).
 
 Optionally, you need:

configure.ac

 AC_PROG_MAKE_SET
 AC_PROG_LIBTOOL
 
+dnl ================================================================
+dnl C11 compiler checks.
+dnl ================================================================
+
+AC_LANG_PUSH(C)
+AC_MSG_CHECKING([if $CC supports C11 without any flags])
+AC_COMPILE_IFELSE(
+	[AC_LANG_PROGRAM(
+		[[// test]],
+		[[]])],
+	[AC_MSG_RESULT([yes])
+	 have_c11_native=yes],
+	[AC_MSG_RESULT([no])
+	 have_c11_native=no])
+AC_LANG_POP(C)
+
+AC_LANG_PUSH(C)
+TEMP_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -std=c11"
+AC_MSG_CHECKING([if $CC supports C11 with the -std=c11 flag])
+AC_COMPILE_IFELSE(
+	[AC_LANG_PROGRAM(
+		[[// test]],
+		[[]])],
+	[AC_MSG_RESULT([yes])
+	 have_c11_c11=yes],
+	[AC_MSG_RESULT([no])
+	 have_c11_c11=no])
+CFLAGS="$TEMP_CFLAGS"
+AC_LANG_POP(C)
+
+AC_MSG_CHECKING([if $CC supports C11])
+if test "$have_c11_c11" = yes ; then
+	AC_MSG_RESULT([-std=c11])
+	CFLAGS="$CFLAGS -std=c11"
+elif test "$have_c11_native" = yes ; then
+	AC_MSG_RESULT([yes])
+else
+	AC_MSG_RESULT([no])
+	AC_MSG_ERROR([C11 is not supported by $CC.])
+fi
+
 dnl ================================================================
 dnl getopt checks.
 dnl ================================================================
 
         Source code location:          ${srcdir}
 
-        Compiler:                      ${CXX}
-        Compiler flags:                ${CXXFLAGS}
+        C11 Compiler:                  ${CC}
+        C11 Compiler flags:            ${CFLAGS}
+
+        C++ Compiler:                  ${CXX}
+        C++ Compiler flags:            ${CXXFLAGS}
 
         wxWidgets:                     ${WX_VERSION} (${WX_CONFIG})
         pulseaudio:                    ${have_pulseaudio}