use -Wno-unused-but-set-variable only if compiler knows it

since APPLE's clang warns about it on every file.

src/Makefile

@@ -8,7 +8,6 @@ INC=-Ilibwps -I. -Ilwe
 
 DISABLED_WARNINGS= \
  -Wno-unused-variable \
- -Wno-unused-but-set-variable \
  -Wno-unused-function \
  -Wno-pointer-sign
 

src/configure

@@ -2018,6 +2018,7 @@ ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
+
 ac_config_files="$ac_config_files config.mak"
 
 ac_ext=c
@@ -3300,6 +3301,46 @@ if test "${enable_savetocurrent+set}" = set; then :
 fi
 
 
+DESIRED_FLAGS="-Wno-unused-but-set-variable"
+for flag in $DESIRED_FLAGS; do
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking to see if compiler understands $flag" >&5
+$as_echo_n "checking to see if compiler understands $flag... " >&6; }
+
+  save_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $flag"
+
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  flag_ok=yes
+else
+  flag_ok=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  CFLAGS="$save_CFLAGS"
+
+  if test "X$flag_ok" = Xyes ; then
+    CFLAGS="$CFLAGS $flag"
+    true
+  else
+
+    true
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $flag_ok" >&5
+$as_echo "$flag_ok" >&6; }
+
+done
+
 cp confdefs.h config.h
 
 target=$PACKAGE_NAME

src/configure.ac

@@ -1,4 +1,5 @@
 AC_INIT(reaver, 1.6.3)
+AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_FILES([config.mak])
 AC_PROG_CC
 AC_LANG(C)
@@ -15,6 +16,11 @@ AC_HELP_STRING([--enable-savetocurrent],
   [saves the current session file to the directory reaver was started from])
 , [ CFLAGS="$CFLAGS -DSAVETOCURRENT" ])
 
+DESIRED_FLAGS="-Wno-unused-but-set-variable"
+for flag in $DESIRED_FLAGS; do
+  AS_COMPILER_FLAG([$flag], [CFLAGS="$CFLAGS $flag"])
+done
+
 cp confdefs.h config.h
 
 AC_SUBST(target, $PACKAGE_NAME)

src/m4/as-compiler-flag.m4

@@ -0,0 +1,64 @@
+dnl as-compiler-flag.m4 0.1.0
+
+dnl autostars m4 macro for detection of compiler flags
+
+dnl David Schleef <ds@schleef.org>
+dnl Tim-Philipp Müller <tim centricular net>
+
+dnl AS_COMPILER_FLAG(CFLAGS, ACTION-IF-ACCEPTED, [ACTION-IF-NOT-ACCEPTED])
+dnl Tries to compile with the given CFLAGS.
+dnl Runs ACTION-IF-ACCEPTED if the compiler can compile with the flags,
+dnl and ACTION-IF-NOT-ACCEPTED otherwise.
+
+AC_DEFUN([AS_COMPILER_FLAG],
+[
+  AC_MSG_CHECKING([to see if compiler understands $1])
+
+  save_CFLAGS="$CFLAGS"
+  CFLAGS="$CFLAGS $1"
+
+  AC_TRY_COMPILE([ ], [], [flag_ok=yes], [flag_ok=no])
+  CFLAGS="$save_CFLAGS"
+
+  if test "X$flag_ok" = Xyes ; then
+    $2
+    true
+  else
+    $3
+    true
+  fi
+  AC_MSG_RESULT([$flag_ok])
+])
+
+dnl AS_CXX_COMPILER_FLAG(CPPFLAGS, ACTION-IF-ACCEPTED, [ACTION-IF-NOT-ACCEPTED])
+dnl Tries to compile with the given CPPFLAGS.
+dnl Runs ACTION-IF-ACCEPTED if the compiler can compile with the flags,
+dnl and ACTION-IF-NOT-ACCEPTED otherwise.
+
+AC_DEFUN([AS_CXX_COMPILER_FLAG],
+[
+  AC_REQUIRE([AC_PROG_CXX])
+
+  AC_MSG_CHECKING([to see if c++ compiler understands $1])
+
+  save_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS $1"
+
+  AC_LANG_PUSH(C++)
+
+  AC_TRY_COMPILE([ ], [], [flag_ok=yes], [flag_ok=no])
+  CPPFLAGS="$save_CPPFLAGS"
+
+  if test "X$flag_ok" = Xyes ; then
+    $2
+    true
+  else
+    $3
+    true
+  fi
+
+  AC_LANG_POP(C++)
+
+  AC_MSG_RESULT([$flag_ok])
+])
+