Work around Mac's broken sed

sed on Mac is apparently incredibly broken and incapable of adding
newlines.  `printf 'x' | sed 's/x/\n/'` results in 'n' on Mac rather
than a newline.  GNU sed even in --posix mode does not agree with this
behavior.  None the less, we can work around it here.

configure.ac

@@ -602,7 +602,7 @@ PLATFORM_CORE_LDFLAGS=
 PLATFORM_CORE_LIBS=
 
 path_remove () {
-  echo "$1" | sed 's/:/\n/g' | grep -Fxv "$2" | tr '\n' ':' | sed 's/:$/\n/'
+  echo "$1" | tr ':' '\n' | grep -Fxv "$2" | tr '\n' ':' | sed 's/:$//'
 }
 path_push_unique () {
   x="$(eval echo \$$1)"