circleci: Escape spaces instead of quoting them

It turns out, in addition to beeing enclosed inside sh -c '...' the
CFLAGS may also be nested inside "double" quotes.

.circleci/config.yml

@@ -365,7 +365,7 @@ workflows:
           name: distcheck_ubuntu_bionic
           dist: ubuntu
           release: bionic
-          extra_conf: CFLAGS="-g -O2 -m32"
+          extra_conf: CFLAGS=-g\ -O2\ -m32
       - distcheck:
           name: distcheck_alpine
           dist: alpine