make default CMAKE_BUILD_TYPE == RELEASE

[h-2]

This behaviour was lost when giving up the "seqan build system".

Add:

if (NOT CMAKE_BUILD_TYPE)
 set (CMAKE_BUILD_TYPE Release CACHE STRING
 "Choose the type of build, options are: Debug Release RelWithDebInfo ..."
 FORCE)
endif (NOT CMAKE_BUILD_TYPE)

see also #70

[h-2]

via e99a1a1