Disable link-time, whole program optimization

When using LTO, the linker calls back to the compiler to figure out
where a certain symbol is defined. However, in many versions there
is apparently a bug that is triggered if a template in inlined in
both a used library and in the main program, but with different
versions of the compiler. The Boost exception class is particular
prone for this.

Thus, we disable the -lto from the development builds by default;
if anyone wants to test of the behaviour of their own compiler, they
must now explicitly turn on this flag.
Include flag for enabling link-time optimizations

When LTO is disabled by default, we need a flag to re-enable it.
Merge pull request #258 from rolk/258_nolto

Disable link-time, whole program optimization