cmake: fix vmaf link w/-DBUILD_SHARED_LIBS=1

Add target_link_libraries(aom_static). This target was added in:
0f23ada000 cmake: Fix aom linkage race condition on MSVC

This target is used to link the examples and tests and was missing the
vmaf library, resulting in undefined symbols.

Also remove -static with -DBUILD_SHARED_LIBS=0 as this may fail if only
the shared object files are available, e.g., in the vmaf package on Arch
Linux.

Bug: aomedia:3464
Fixed: aomedia:3464
Change-Id: I314f961bfaa3819c4474d1a659e1c1575b58206c
(cherry picked from commit 59f32b622ee313eb0cd501a6cffdee54223aff10)

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 371ecd6..2210905 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -640,12 +640,10 @@
     if(PKG_CONFIG_FOUND)
       pkg_check_modules(VMAF REQUIRED libvmaf)
       if(BUILD_SHARED_LIBS)
-        target_link_libraries(aom PRIVATE ${VMAF_LDFLAGS} ${VMAF_LIBRARIES})
-      else()
-        target_link_libraries(aom
-                              PRIVATE ${VMAF_LDFLAGS} ${VMAF_LIBRARIES} -static)
+        target_link_libraries(aom_static
+                              PRIVATE ${VMAF_LDFLAGS} ${VMAF_LIBRARIES})
       endif()
-      target_include_directories(aom PRIVATE ${VMAF_INCLUDE_DIRS})
+      target_link_libraries(aom PRIVATE ${VMAF_LDFLAGS} ${VMAF_LIBRARIES})
       target_include_directories(aom_dsp_encoder PRIVATE ${VMAF_INCLUDE_DIRS})
       if(VMAF_CFLAGS)
         foreach(flag "${VMAF_CFLAGS}")