gitlab-ci: Add default cmake and cmake-nohttp jobs

In addition to testing the build with a legacy cmake-3.16 upstream
package, install the runner's default cmake version and test against
that for broader coverage. On the current `gcc:9` container image
this is cmake 3.18.4.

Also test build with `-D OP_DISABLE_HTTP=YES` to confirm the build
completes with this option, if not that it does what it's supposed
to. This should protect against regression of the earlier fix.

Author: rillian

.gitlab-ci.yml

@@ -16,7 +16,7 @@ autotools:
     - make
     - make distcheck
 
-cmake:
+cmake-3.16:
   stage: build
   before_script:
     - apt-get update &&
@@ -30,6 +30,30 @@ cmake:
   tags:
     - docker
 
+cmake:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y libopus-dev libogg-dev libssl-dev
+        cmake doxygen
+  script:
+    - cmake -Bbuild -H.
+    - cmake --build build
+  tags:
+    - docker
+
+cmake-nohttp:
+  stage: build
+  before_script:
+    - apt-get update &&
+      apt-get install -y libopus-dev libogg-dev libssl-dev
+        cmake doxygen
+  script:
+    - cmake -Bbuild -H. -D OP_DISABLE_HTTP=YES
+    - cmake --build build
+  tags:
+    - docker
+
 makefile:
   stage: build
   before_script: