Add CircleCI gcc-9 build

.circleci/config.yml

@@ -43,6 +43,30 @@ jobs:
             ls -l nnn
             make clean
 
+  gcc-9:
+    docker:
+      - image: ubuntu:18.04
+        working_directory: ~/nnn
+        environment:
+          CC: gcc-9
+          CI_FORCE_TEST: 1
+    steps:
+      - run:
+          command: |
+            apt update -qq
+            apt install -y --no-install-recommends software-properties-common
+            apt-add-repository -y ppa:jonathonf/gcc-9.0
+            apt update -qq
+            apt install -y --no-install-recommends git gcc-9 make pkg-config libncursesw5-dev libreadline-dev
+      - checkout
+      - run:
+          command: |
+            export CFLAGS=-Werror
+            make clean
+            make strip
+            ls -l nnn
+            make clean
+
   package-and-publish:
     machine: true
     working_directory: ~/nnn
@@ -67,6 +91,7 @@ workflows:
     jobs: &all-tests
       - gcc
       - gcc-8
+      - gcc-9
 
   nightly:
     triggers: