cmake: add missing defaults for PPC.

Change-Id: Iff02efe3d8b1ce029bdccb6a34de4694ab4de0e4
diff --git a/build/cmake/aom_config_defaults.cmake b/build/cmake/aom_config_defaults.cmake
index 3990e0b..e638fcf 100644
--- a/build/cmake/aom_config_defaults.cmake
+++ b/build/cmake/aom_config_defaults.cmake
@@ -18,6 +18,7 @@
 # CPUs.
 set(ARCH_ARM 0 CACHE NUMBER "Enables ARM architecture.")
 set(ARCH_MIPS 0 CACHE NUMBER "Enables MIPS architecture.")
+set(ARCH_PPC 0 CACHE NUMBER "Enables PPC architecture.")
 set(ARCH_X86 0 CACHE NUMBER "Enables X86 architecture.")
 set(ARCH_X86_64 0 CACHE NUMBER "Enables X86_64 architecture.")
 
@@ -31,6 +32,9 @@
 set(HAVE_MIPS64 0 CACHE NUMBER "Enables MIPS64 optimizations. ")
 set(HAVE_MSA 0 CACHE NUMBER "Enables MSA optimizations.")
 
+# PPC optimization flags.
+set(HAVE_VSX 0 CACHE NUMBER "Enables VSX optimizations.")
+
 # x86/x86_64 optimization flags.
 set(HAVE_AVX 0 CACHE NUMBER "Enables AVX optimizations.")
 set(HAVE_AVX2 0 CACHE NUMBER "Enables AVX2 optimizations.")