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>

src/gallium/drivers/swr/Makefile.am

@@ -246,6 +246,7 @@ if HAVE_SWR_AVX
 lib_LTLIBRARIES += libswrAVX.la
 
 libswrAVX_la_CXXFLAGS = \
+	$(PTHREAD_CFLAGS) \
 	$(SWR_AVX_CXXFLAGS) \
 	-DKNOB_ARCH=KNOB_ARCH_AVX \
 	$(COMMON_CXXFLAGS)
@@ -253,6 +254,9 @@ libswrAVX_la_CXXFLAGS = \
 libswrAVX_la_SOURCES = \
 	$(COMMON_SOURCES)
 
+libswrAVX_la_LIBADD = \
+	$(PTHREAD_LIBS)
+
 libswrAVX_la_LDFLAGS = \
 	$(COMMON_LDFLAGS)
 endif
@@ -260,6 +264,7 @@ endif
 if HAVE_SWR_AVX2
 lib_LTLIBRARIES += libswrAVX2.la
 libswrAVX2_la_CXXFLAGS = \
+	$(PTHREAD_CFLAGS) \
 	$(SWR_AVX2_CXXFLAGS) \
 	-DKNOB_ARCH=KNOB_ARCH_AVX2 \
 	$(COMMON_CXXFLAGS)
@@ -267,6 +272,9 @@ libswrAVX2_la_CXXFLAGS = \
 libswrAVX2_la_SOURCES = \
 	$(COMMON_SOURCES)
 
+libswrAVX2_la_LIBADD = \
+	$(PTHREAD_LIBS)
+
 libswrAVX2_la_LDFLAGS = \
 	$(COMMON_LDFLAGS)
 endif