MDEV-21942: Correctly use newer libpcre2 version (regression in b6b69806)

If libpcre2 (10.x) is not defined and only the older libpcre3 (8.x)
would be available, the build would attempt to download it from the
Internet, violating many build environment restrictions.

Commit b6b69806 attempted to fix
MDEV-21942 but was incomplete, and after in CI systems started
failing since the .travis.yml update was missing.

.travis.yml

@@ -51,7 +51,7 @@ addons:
       - libjudy-dev
       - libncurses5-dev
       - libpam0g-dev
-      - libpcre3-dev
+      - libpcre2-dev
       - libreadline-gplv2-dev
       - libstemmer-dev
       - libssl-dev

debian/autobake-deb.sh

@@ -117,6 +117,13 @@ fi
 
 sed -i -e "/Package: mariadb-plugin-tokudb/,/^$/d" debian/control
 
+# If libpcre2-dev is not available (before Debian Stretch and Ubuntu Xenial)
+# attempt to build using older libpcre3-dev (SIC!)
+if ! apt-cache madison libpcre2-dev | grep --quiet 'libpcre2-dev'
+then
+  sed 's/libpcre2-dev/libpcre3-dev/' -i debian/control
+fi
+
 # Mroonga, TokuDB never built on Travis CI anyway, see build flags above
 if [[ $TRAVIS ]]
 then