Don't search for disabled packages

Setting CMAKE_DISABLE_FIND_PACKAGE_xxx flag should make find_package
not doing anything, but this does not seem to work with older versions
of CMake. Anyway, we can test this flag and make sure that we don't
call find_package in that case.

Also includes a check for both upper- and same-case versions of found
flag, as some modules, most notably Boost, does not set the upper-case
version of their _FOUND flag; this tests for both.
Merge pull request #327 from rolk/327_disable

Don't search for disabled packages