gitlab-ci: allow enabling/disabling building documentation in "nm-ci-run.sh"

g-ir-scanner does not support building with clang, due to [1], [2], [3].

It triggers

    checking if /usr/bin/g-ir-scanner works... no (compiler failure -- check config.log)
    configure: error: introspection enabled but can't be used

with

    clang-7: error: unknown argument: '-fstack-clash-protection'

See also commit 99b92fd992, which adds this configure
check.

Honor the environment variable WITH_DOCS to allow the caller to overwrite
the automatic detection that the script does.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=757934
[2] https://gitlab.gnome.org/GNOME/gobject-introspection/issues/150
[3] c14d037228

contrib/scripts/nm-ci-run.sh

@@ -5,6 +5,7 @@
 #  - CC
 #  - BUILD_TYPE
 #  - CFLAGS
+#  - WITH_DOCS
 
 set -exv
 
@@ -13,6 +14,20 @@ die() {
     exit 1
 }
 
+_is_true() {
+    case "$1" in
+        1|y|yes|YES|Yes|on)
+            return 0
+            ;;
+        0|n|no|NO|No|off)
+            return 1
+            ;;
+        *)
+            die "not a boolean argument \"$1\""
+            ;;
+    esac
+}
+
 ###############################################################################
 
 if [ "$BUILD_TYPE" == meson ]; then
@@ -45,6 +60,14 @@ if [ "$CC" != gcc ]; then
     _WITH_CRYPTO=nss
 fi
 
+if [ "$WITH_DOCS" != "" ]; then
+    if _is_true "$WITH_DOCS"; then
+        _WITH_DOCS="$_TRUE"
+    else
+        _WITH_DOCS="$_FALSE"
+    fi
+fi
+
 ###############################################################################
 
 _autotools_test_print_logs() {