Clang aims to be compatible with GCC when it comes to command-line
parameters. Where we enable functionality based on the presence of
a GCC-compiler, we can use the same functionality with Clang.
This patch introduces a new variable CXX_COMPAT_GCC which is true if
the compiler handles the same options as GCC, and this variable is
subsequently used in tests instead of CMAKE_COMPILER_IS_GNUCXX (which
remains if we need to test if we really have GCC, e.g. for version)
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.
