Use -Bsymbolic for shared libs only, not executables

Some linkers fail if this flag is used when creating an executable file.
Since the flag is meaningful only for shared libraries, dropping it from
the main link flags is correct.

configure

@@ -2899,7 +2899,7 @@ enabled extra_warnings && check_cflags -Winline
 check_ldflags -Wl,--warn-common
 check_ldflags -Wl,--as-needed
 check_ldflags '-Wl,-rpath-link,\$(BUILD_ROOT)/libpostproc -Wl,-rpath-link,\$(BUILD_ROOT)/libswscale -Wl,-rpath-link,\$(BUILD_ROOT)/libavfilter -Wl,-rpath-link,\$(BUILD_ROOT)/libavdevice -Wl,-rpath-link,\$(BUILD_ROOT)/libavformat -Wl,-rpath-link,\$(BUILD_ROOT)/libavcodec -Wl,-rpath-link,\$(BUILD_ROOT)/libavcore -Wl,-rpath-link,\$(BUILD_ROOT)/libavutil'
-check_ldflags -Wl,-Bsymbolic
+test_ldflags -Wl,-Bsymbolic && append SHFLAGS -Wl,-Bsymbolic
 
 echo "X{};" > $TMPV
 if test_ldflags -Wl,--version-script,$TMPV; then