doxy: fix conditional usage, ref warnings

doxygen < 1.7.? seems to have been more tolerant of single line
\if/\endif

This change fixes warnings such as:
mainpage.dox:13: warning: unable to resolve reference to `vp8_encoder-'
for \ref command
vpx_decoder.h:193: warning: explicit link request to 'n' could not be
resolved

Change-Id: If3d04af5ede1b0d1e2c63021d0e4ac8f98db20b2