build: Remove spaces from variable-printing rules

This simplifies parsing when using these rules from scripts.

contrib/guix/guix-build

@@ -119,7 +119,7 @@ fi
 for host in $HOSTS; do
     case "$host" in
         *darwin*)
-            OSX_SDK="$(make -C "${PWD}/depends" --no-print-directory HOST="$host" print-OSX_SDK | sed 's@^[^=]\+=[[:space:]]\+@@g')"
+            OSX_SDK="$(make -C "${PWD}/depends" --no-print-directory HOST="$host" print-OSX_SDK | sed 's@^[^=]\+=@@g')"
             if [ -e "$OSX_SDK" ]; then
                 echo "Found macOS SDK at '${OSX_SDK}', using..."
             else