Update to new out-of-source build mechanism

aom.spec

@@ -1,3 +1,6 @@
+# Force out of source build
+%undefine __cmake_in_source_build
+
 %global sover           2
 # git describe
 %global aom_version     v2.0.0
@@ -71,7 +74,7 @@ sed -i 's@set(aom_version "")@set(aom_version "%{aom_version}")@' build/cmake/ve
 sed -i 's@libvmaf\.a @@' CMakeLists.txt
 
 %build
-%cmake3 -B _build -DENABLE_CCACHE=1 \
+%cmake3 -DENABLE_CCACHE=1 \
         -DCMAKE_SKIP_RPATH=1 \
         -DCMAKE_BUILD_TYPE=RelWithDebInfo \
         -DCONFIG_WEBM_IO=1 \
@@ -85,10 +88,10 @@ sed -i 's@libvmaf\.a @@' CMakeLists.txt
         -DCONFIG_TUNE_VMAF=1 \
 %endif
         %{nil}
-%make_build -C _build
+%cmake3_build
 
 %install
-%make_install -C _build
+%cmake3_install
 rm -rf %{buildroot}%{_libdir}/libaom.a
 
 %files