refactor(src/CMakeLists.txt): simplify conditions in if
refactor(src/CMakeLists.txt): simplify conditions in if

There are many places where we do not need 'MATCHES "ON"' or 'MATCHES
"OFF"'. Reference: http://www.cmake.org/cmake/help/v3.0/command/if.html

~~~
if(<constant>)

True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number. False
if the constant is 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty
string, or ends in the suffix -NOTFOUND. Named boolean constants are
case-insensitive. If the argument is not one of these constants, it is
treated as a variable.
~~~

leanprover-community/lean,Kha/lean,avigad/lean,sp3ctum/lean,UlrikBuchholtz/lean,c-cube/lean,fgdorais/lean,digama0/lean,htzh/lean,rlewis1988/lean,soonhokong/lean-osx,Kha/lean,UlrikBuchholtz/lean,eigengrau/lean,levnach/lean,UlrikBuchholtz/lean,levnach/lean,javra/lean,sp3ctum/lean,fpvandoorn/lean,javra/lean,UlrikBuchholtz/lean,fpvandoorn/lean2,UlrikBuchholtz/lean,leanprover/lean,digama0/lean,soonhokong/lean-windows,eigengrau/lean,leanprover-community/lean,eigengrau/lean,htzh/lean,dselsam/lean,avigad/lean,leanprover-community/lean,sp3ctum/lean,javra/lean,Kha/lean,fgdorais/lean,rlewis1988/lean,fpvandoorn/lean2,fpvandoorn/lean,digama0/lean,digama0/lean,leanprover/lean,johoelzl/lean,levnach/lean,sp3ctum/lean,avigad/lean,leanprover-community/lean,leanprover-community/lean,Kha/lean,avigad/lean,leodemoura/lean,fpvandoorn/lean,soonhokong/lean,leodemoura/lean,rlewis1988/lean,fgdorais/lean,fpvandoorn/lean2,soonhokong/lean-osx,c-cube/lean,leanprover/lean,leanprover-community/lean,dselsam/lean,leodemoura/lean,soonhokong/lean,fpvandoorn/lean2,dselsam/lean,soonhokong/lean-windows,htzh/lean,fgdorais/lean,eigengrau/lean,soonhokong/lean-windows,levnach/lean,dselsam/lean,johoelzl/lean,johoelzl/lean,levnach/lean,soonhokong/lean-osx,soonhokong/lean-osx,leanprover/lean,johoelzl/lean,fpvandoorn/lean,rlewis1988/lean,leodemoura/lean,avigad/lean,leodemoura/lean,leanprover/lean,dselsam/lean,fgdorais/lean,Kha/lean,soonhokong/lean,digama0/lean,c-cube/lean,soonhokong/lean-osx,fpvandoorn/lean2,dselsam/lean,soonhokong/lean,javra/lean,johoelzl/lean,javra/lean,Kha/lean,sp3ctum/lean,soonhokong/lean,htzh/lean,rlewis1988/lean,c-cube/lean,fpvandoorn/lean,leodemoura/lean,fpvandoorn/lean,soonhokong/lean-windows,c-cube/lean,digama0/lean,leanprover/lean,avigad/lean,eigengrau/lean,johoelzl/lean,htzh/lean,soonhokong/lean-windows,fgdorais/lean,rlewis1988/lean