Add CFLAGS to the linker step

The GCC manual states for different parameters that the options for compilation
must also be used when linking. The options for compilation are stored in
CFLAGS and added to LINK.o to fix the behavior.

Option which need this are for example -fPIC/-fPIE or -flto.
Enable Link-time optimization by default

The GCC introduced Link-time optimization in GCC 4.5 (2010-04-14). This should
be long enough available that interested users have upgraded to a compiler
supporting it.

The MSVC project already enabled WholeProgramOptimization since a long time.
Enabling it by default in GCC seems to be equally valid.
Allow to statically set SDL_(CFLAGS|LIBS) without sdl-config

n.pepinpe requested in #540 to allow overwriting the automatically searched
CFLAGS and LIBS for used libraries. This should allow distributors to set the
build configuration without patching the makefile. This is important for
systems were pkg-config or sdl-config aren't normally used.