swr: don't forget to link AVX/AVX2 against pthreads

Seems like the backends have been using pthreads since day one, yet
we've been missing the link.

With later commit we'll fix a typo, hence the libraries will be build
with -Wl,no-undefined, aka failing the build on unresolved symbols.

v2: Split from a larger patch.

Cc: mesa-stable@lists.freedesktop.org
Cc: Bruce Cherniak <bruce.cherniak@intel.com>
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Laurent Carlier <lordheavym@gmail.com>
Fixes: c6e67f5a93 "gallium/swr: add OpenSWR rasterizer"
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 33d397ada5)
[Emil Velikov: add PTHREAD_LIBS to COMMON_LIBADD]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/gallium/drivers/swr/Makefile.am

src/gallium/drivers/swr/Makefile.am

@@ -158,7 +158,8 @@ rasterizer/core/gen_BackendPixelRate0.cpp: rasterizer/codegen/gen_backends.py ra
 COMMON_LIBADD = \
 	$(top_builddir)/src/gallium/auxiliary/libgallium.la \
 	$(top_builddir)/src/mesa/libmesagallium.la \
-	$(LLVM_LIBS)
+	$(LLVM_LIBS) \
+	$(PTHREAD_LIBS)
 
 COMMON_LDFLAGS = \
 	-shared \
@@ -172,6 +173,7 @@ COMMON_LDFLAGS = \
 lib_LTLIBRARIES = libswrAVX.la libswrAVX2.la
 
 libswrAVX_la_CXXFLAGS = \
+	$(PTHREAD_CFLAGS) \
 	$(SWR_AVX_CXXFLAGS) \
 	-DKNOB_ARCH=KNOB_ARCH_AVX \
 	$(COMMON_CXXFLAGS)
@@ -186,6 +188,7 @@ libswrAVX_la_LDFLAGS = \
 	$(COMMON_LDFLAGS)
 
 libswrAVX2_la_CXXFLAGS = \
+	$(PTHREAD_CFLAGS) \
 	$(SWR_AVX2_CXXFLAGS) \
 	-DKNOB_ARCH=KNOB_ARCH_AVX2 \
 	$(COMMON_CXXFLAGS)