docs in gst-plugins-bad fails to build if no gl dependencies are installed.

The external dependency is in:

https://cgit.freedesktop.org/gstreamer/gst-plugins-bad/tree/docs/libs/Makefile.am#n64

The error:
```
-*- mode: compilation; default-directory: "~/gst/master/gst-plugins-bad/docs/libs/" -*-
Compilation started at Thu Oct 19 12:43:39

make
 cd ../.. && /bin/bash /home/francisv/gst/master/gst-plugins-bad/missing automake-1.15 --gnu docs/libs/Makefile
configure.ac:71: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:71: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:71: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.
 cd ../.. && /bin/bash ./config.status docs/libs/Makefile 
config.status: creating docs/libs/Makefile
  DOC   Scanning header files
  DOC   Introspecting gobjects
libtool:   error: cannot find the library '../../gst-libs/gst/gl/libgstgl-1.0.la' or unhandled argument '../../gst-libs/gst/gl/libgstgl-1.0.la'
Linking of scanner failed: 
Makefile:1118: recipe for target 'scan-build.stamp' failed
make: *** [scan-build.stamp] Error 1

Compilation exited abnormally with code 2 at Thu Oct 19 12:43:42

```


The quick fix below brings up other reference issues

if USE_GL
  GTKDOC_LIBS += $(top_builddir)/gst-libs/gst/gl/libgstgl-@GST_API_VERSION@.la
endif

The error:

```
-*- mode: compilation; default-directory: "~/gst/master/gst-plugins-bad/docs/libs/" -*-
Compilation started at Thu Oct 19 12:46:31

make
 cd ../.. && /bin/bash /home/francisv/gst/master/gst-plugins-bad/missing automake-1.15 --gnu docs/libs/Makefile
configure.ac:71: warning: The 'AM_PROG_MKDIR_P' macro is deprecated, and its use is discouraged.
configure.ac:71: You should use the Autoconf-provided 'AC_PROG_MKDIR_P' macro instead,
configure.ac:71: and use '$(MKDIR_P)' instead of '$(mkdir_p)'in your Makefile.am files.
 cd ../.. && /bin/bash ./config.status docs/libs/Makefile 
config.status: creating docs/libs/Makefile
  DOC   Scanning header files
  DOC   Introspecting gobjects
.libs/gst-plugins-bad-libs-scan.o: In function `get_object_types':
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:50: undefined reference to `gst_gl_allocation_params_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:51: undefined reference to `gst_gl_base_memory_allocator_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:52: undefined reference to `gst_gl_buffer_allocation_params_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:53: undefined reference to `gst_gl_buffer_allocator_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:54: undefined reference to `gst_gl_video_allocation_params_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:55: undefined reference to `gst_gl_memory_allocator_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:56: undefined reference to `gst_gl_memory_pbo_allocator_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:57: undefined reference to `gst_gl_renderbuffer_allocation_params_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:58: undefined reference to `gst_gl_renderbuffer_allocator_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:59: undefined reference to `gst_gl_base_filter_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:60: undefined reference to `gst_gl_buffer_pool_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:61: undefined reference to `gst_gl_color_convert_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:62: undefined reference to `gst_gl_context_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:63: undefined reference to `gst_gl_display_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:64: undefined reference to `gst_gl_filter_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:65: undefined reference to `gst_gl_framebuffer_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:66: undefined reference to `gst_gl_shader_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:67: undefined reference to `gst_glsl_stage_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:68: undefined reference to `gst_gl_overlay_compositor_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:69: undefined reference to `gst_gl_upload_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:70: undefined reference to `gst_gl_view_convert_get_type'
/home/francisv/gst/master/gst-plugins-bad/docs/libs/gst-plugins-bad-libs-scan.c:71: undefined reference to `gst_gl_window_get_type'
collect2: error: ld returned 1 exit status
Linking of scanner failed: 
Makefile:1115: recipe for target 'scan-build.stamp' failed
make: *** [scan-build.stamp] Error 1

Compilation exited abnormally with code 2 at Thu Oct 19 12:46:35
```