configure: add a check for gjs (JavaScript interpreter with GI binding).

As discussed on IRC, I am not sure if really this is needed, since
theoretically we now have support for a whole bunch of languages without
doing anything (languages supported by GObject Introspection) and we are
not going to check for all of them.
Yet since now we install one JS plug-in, let's still do a quick check.
We'll see later if we want to change how we handle supported script
languages.

configure.ac

@@ -2313,6 +2313,51 @@ fi
 AM_CONDITIONAL(HAS_PYTHON_INTERP, test "x$PYBIN_PATH" != "x")
 AM_CONDITIONAL(BUILD_PYTHON, test "x$with_python" != xno)
 
+###########################################
+# Check for JavaScript runtime dependencies
+###########################################
+
+AC_ARG_WITH(javascript,
+            [  --with-javascript=no|yes|force install JavaScript plug-ins (default=yes)], ,
+            [with_javascript=yes])
+
+if test "x$with_javascript" = xno; then
+  warning_javascript="
+WARNING: you disabled the installation of core JavaScript plug-ins. This
+         is discouraged as it won't provide the full GIMP experience.
+         Note that you may install the JavaScript plug-ins even if the
+         GJS interpreter is not installed at build-time by passing the
+         configure option --with-javascript=force.
+         Just make sure that GJS is available at run-time."
+elif test "x$with_javascript" = xforce; then
+  with_javascript="yes"
+  warning_javascript=
+  AC_CHECK_PROGS(GJS, gjs, no)
+  if test "x$GJS" = xno; then
+    warning_javascript="
+WARNING: GJS not found.
+         JavaScript plug-ins will be installed anyway but you should make
+         sure that the JavaScript interpreter GJS is available at
+         installation, otherwise installed plug-ins won't be usable."
+    with_javascript="yes (see warning below)"
+  fi
+else
+  with_javascript="yes"
+  AC_CHECK_PROGS(GJS, gjs, no)
+  if test "x$GJS" = xno; then
+    required_deps="$required_deps
+    - GJS not found.
+        *** Please install the JavaScript interpreter GJS.
+        *** Note that you may install the JavaScript plug-ins even if
+        *** GJS is not installed at build-time by passing the configure
+        *** option --with-javascript=force.
+        *** Just make sure that GJS is available at run-time."
+    with_javascript="no"
+  fi
+fi
+
+AM_CONDITIONAL(HAS_JAVASCRIPT_INTERP, test "x$GJS" != "xno")
+AM_CONDITIONAL(BUILD_JAVASCRIPT, test "x$with_javascript" != xno)
 
 ###########################################################
 # Some plug-ins don't build on Win32, others are Win32-only
@@ -3046,6 +3091,7 @@ Optional Plug-Ins:
   PDF (export):              $have_cairo_pdf
   Print:                     $enable_print
   Python 3 plug-ins:         $with_python
+  JavaScript plug-ins:       $with_javascript
   TWAIN (Win32):             $os_win32
   Webpage:                   $have_webkit
   WMF:                       $have_libwmf
@@ -3064,7 +3110,7 @@ Tests:
   Test desktop file          $have_desktop_file_validate
 
 Bug report URL: $with_bug_report_url
-$override_bug_report_url$warning_vector_icons_windows$warning_glib_networking$warning_gcc$warning_python"
+$override_bug_report_url$warning_vector_icons_windows$warning_glib_networking$warning_gcc$warning_python$warning_javascript"
 
 if test "x$required_deps" = "x"; then
   AC_OUTPUT

plug-ins/goat-exercises/Makefile.am

@@ -62,12 +62,16 @@ goat_exercise_c_LDADD = \
 
 # Javascript (GJS) version.
 
+if BUILD_JAVASCRIPT
 goat_exercise_gjsdir = $(gimpplugindir)/plug-ins/goat-exercise-gjs
 goat_exercise_gjs_SCRIPTS = goat-exercise-gjs.js
+endif
 
 # Python 3 (pygobject) version.
 
 # Commented out for now until we figure out the GParamSpec issues.
 
+if BUILD_PYTHON
 #goat_exercise_py3dir = $(gimpplugindir)/plug-ins/goat-exercise-py3
 #goat_exercise_py3_SCRIPTS = goat-exercise-py3.py
+endif