Quotes added on configure script's string comparison.

Solves #17744

The syntax has been generalized too, so there are only double quotes.

Signed-off-by: zerosion <zerosion@protonmail.ch>

changes/ticket17744

+  o Code simplification and refactoring:
+    - Quote all the string interpolations in configure.ac -- even
+      those which we are pretty sure can't contain spaces. Closes
+      ticket 17744. Patch from "zerosion".
\ No newline at end of file