pkgconfig values escape paths after meson generation port

a28a7565 switched the way pkgconfig files are generated to use the meson pkgconfig module.

This caused some of our .pc files to now include a slash to escape the whitespace. Meson escapes the paths

gstreamer-plugins-base-1.0.pc
...
13:libraries=allocators\ app\ audio\ fft\ pbutils\ riff\ rtp\ rtsp\ sdp\ tag\ video\ gl

gstreamer-gl-1.0.pc
13:gl_platforms=egl\ glx
14:gl_winsys=wayland\ x11
15:gl_apis=gles2\ gl

Compared to how they are installed up to 1.18:

gstreamer-plugins-base-1.0.pc
...
libraries=allocators app audio fft pbutils riff rtp rtsp sdp tag video gl
gstreamer-gl-1.0.pc

...
gl_platforms=egl glx
gl_winsys=wayland x11
gl_apis=gles2 gl

pkg-config returns the escaped path as is when called, which now causes issues with building gst-plugins-good. We are trying to match the exact strings, but now they have a \ appended so we fail to enable the GL plugins.

pkg-config gstreamer-gl-1.0 --variable gl_platforms
egl\ glx

-good building against the new .pc files

Message: GStreamer OpenGL window systems: wayland\ x11\ gbm
Message: GStreamer OpenGL platforms: egl\ glx
Message: GStreamer OpenGL apis: gles2\ gl
Run-time dependency gstreamer-gl-prototypes-1.0 found: YES 1.19.0.1
Run-time dependency zlib found: YES 1.2.11

-good building from gst-build

Message: GStreamer OpenGL window systems: wayland x11 gbm
Message: GStreamer OpenGL platforms: egl glx
Message: GStreamer OpenGL apis: gles2 gl
Dependency gstreamer-gl-prototypes-1.0 found: YES 1.19.0.1 (overridden)
Dependency gstreamer-gl-x11-1.0 found: YES 1.19.0.1 (overridden)
Dependency gstreamer-gl-wayland-1.0 found: YES 1.19.0.1 (overridden)
Dependency gstreamer-gl-egl-1.0 found: YES 1.19.0.1 (overridden)
Dependency zlib found: YES 1.2.11 (cached)

cc @xclaesse

cc @tpm I think this is a release blocker for 1.20