cmake: fixed boost version test

"if(${Boost_VERSION} EQUAL ${ver})" fixed

if §{Boost_VERSION} is unset, the expression expands to something like
if( EQUAL "104601" )
which generates a parser error. Since it's a string comparison, anyway,
I replaced it by the specific
if("${Boost_VERSION}" STREQUAL ${ver})

fed
Merge branch 'maint'
Merge branch 'master' of github.com:willcode/gnuradio