libXvMC-1.0.12 breaks mesa build

hi

(used configuration is appended below)

building mesa-19.2.0 vs recently released libXvMC-1.0.12 crashes with:

----8<----
/usr/bin/ld: src/gallium/state_trackers/xvmc/libxvmc_st.a(context.c.o): in function `Validate':
/home/balducci/tmp/install-us-d/mesa-19.2.0-rc4.d/mesa-19.2.0/.build/../src/gallium/state_trackers/xvmc/context.c:71: undefined reference to `XvQueryAdaptors'
/usr/bin/ld: /home/balducci/tmp/install-us-d/mesa-19.2.0-rc4.d/mesa-19.2.0/.build/../src/gallium/state_trackers/xvmc/context.c:120: undefined reference to `XvFreeAdaptorInfo'
/usr/bin/ld: /home/balducci/tmp/install-us-d/mesa-19.2.0-rc4.d/mesa-19.2.0/.build/../src/gallium/state_trackers/xvmc/context.c:86: undefined reference to `XvFreeAdaptorInfo'
collect2: error: ld returned 1 exit status
---->8----

Those undefs are contained in libXv; but the Xv library dependency has been removed from xvmc.pc:

----8<----
commit dd9ae03725990ee9c9336bca4b267dfbb696002f
Author: Adam Jackson <ajax@redhat.com>
Date:   Tue Jul 23 15:28:26 2019 -0400

    pkgconfig: Remove xv from xvmc.pc

    This library does not itself require libXv for anything.
---->8----

So -lXv has now to be explicitly pulled in by the configuration process

For me things get fixed by running with LDFLAGS="-lXv", but may be the mesa configuration process should be updated to support the change in xvmc.pc

I'm using the following configuration:

----8<----
--prefix=/opt/stow.d/versions/mesa-19.2.0-llvm-8.0/usr
--libdir=/opt/stow.d/versions/mesa-19.2.0-llvm-8.0/usr/lib64
-Dgles1=true
-Dgles2=true
-Dgallium-xa=true
-Dgbm=true
-Dshared-llvm=true
-Dglx=dri
-Dplatforms="drm,x11,wayland"
-Dgallium-drivers="nouveau,r300,r600,radeonsi,svga,swrast"
-Ddri-drivers="auto,nouveau,swrast"
---->8----

thanks

ciao gabriele