configure: add filtering of linker flags

This allows filtering of linker flags the same was as already
supported for CFLAGS.  The filter must be initialised to 'echo'
early since it is invoked by --extra-ldflags.
Signed-off-by: NMans Rullgard <mans@mansr.com>

configure

@@ -597,11 +597,11 @@ add_asflags(){
 }
 
 add_ldflags(){
-    append LDFLAGS "$@"
+    append LDFLAGS $($ldflags_filter "$@")
 }
 
 add_extralibs(){
-    prepend extralibs "$@"
+    prepend extralibs $($ldflags_filter "$@")
 }
 
 check_cmd(){
@@ -657,6 +657,8 @@ check_ld(){
         test "${f}" = "${f#-l}" && flags="$flags $f" || libs="$libs $f"
     done
     check_cc $($cflags_filter $flags) || return
+    flags=$($ldflags_filter $flags)
+    libs=$($ldflags_filter $libs)
     check_cmd $ld $LDFLAGS $flags -o $TMPE $TMPO $libs $extralibs
 }
 
@@ -1706,6 +1708,8 @@ LIB_INSTALL_EXTRA_CMD='$$(RANLIB) "$(LIBDIR)/$(LIBNAME)"'
 SLIB_INSTALL_NAME='$(SLIBNAME_WITH_VERSION)'
 SLIB_INSTALL_LINKS='$(SLIBNAME_WITH_MAJOR) $(SLIBNAME)'
 
+ldflags_filter=echo
+
 AS_O='-o $@'
 CC_O='-o $@'