configure: ignore usage of C11 _Generic on FreeBSD

BatteryMeter.c:30:8: error: '_Generic' is a C11 extension [-Werror,-Wc11-extensions]
   if (isnan(percent)) {
       ^
/usr/include/math.h:114:2: note: expanded from macro 'isnan'
        __fp_type_select(x, __inline_isnanf, __inline_isnan, __inline_isnanl)
        ^
/usr/include/math.h:82:39: note: expanded from macro '__fp_type_select'
#define __fp_type_select(x, f, d, ld) _Generic((x),                     \
                                      ^
1 error generated.

configure.ac

@@ -531,6 +531,11 @@ AM_CFLAGS="\
  -Wunused\
  -Wwrite-strings"
 
+# FreeBSD uses C11 _Generic in its isnan implementation, even with -std=c99
+if test "$my_htop_platform" = freebsd; then
+   AM_CFLAGS="$AM_CFLAGS -Wno-c11-extensions"
+fi
+
 dnl https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
 AC_DEFUN([AX_CHECK_COMPILE_FLAG],
 [