fix clang build not seeing some libc prototypes throwing build warnings

libs/spandsp/configure.ac

@@ -464,6 +464,13 @@ intel)
     fi
     COMP_VENDOR_LDFLAGS=
     ;;
+clang*)
+    COMP_VENDOR_CFLAGS="-D_XOPEN_SOURCE=700 -std=c99 -Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes $COMP_VENDOR_CFLAGS"
+    if test x"$ac_cv_gcc_unused_but_set_variable" = xyes ; then
+        COMP_VENDOR_CFLAGS="-Wunused-but-set-variable $COMP_VENDOR_CFLAGS"
+    fi
+    COMP_VENDOR_LDFLAGS=
+    ;;
 *)
     COMP_VENDOR_CFLAGS="-std=c99 -Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes $COMP_VENDOR_CFLAGS"
     if test x"$ac_cv_gcc_unused_but_set_variable" = xyes ; then