Boost not found leads to Cmake Error: "if(${Boost_VERSION} EQUAL ${ver})"

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