Use FC instead of CC to link the dynamic library on OS X

Avoids problems of libgfortran not being found.

Remove -Wl,--retain-symbols-file from dynamic link line to fix tool support

The aim is to restrict the symbols exported from openBLAS, but for dynamic libraries --retain-symbols-file has the opposite effect intended. It removes symbols from the .symtab section of the library, which is used by tools such as debuggers, profilers, objdump etc. but does not remove them from .dynsym, which is the section used by the runtime when loading the binary.

In short, using --retain-symbols-file like this doesn't prevent symbol leakage but does prevent tools from analyzing applications linked with openBLAS.

This patch improves tool support for openBLAS applications but does not address symbol leakage through .dynsym.

check if GEMM_MULTITHREAD_THRESHOLD defined in gemm.c
Set a fallback value.

fix default prefix handling in makefiles

get rid of the generated cblas_noconst.h file

PREFIX wasn't communicated to Makefile.install (where it matters)
by Makefile. The result is that the default PREFIX is empty and
OpenBLAS was being installed in /lib.

Avoid failure on qemu guests declaring an Athlon CPU without 3dnow!

The present patch verifies that, on machines declaring an Athlon CPU model and
family, the 3dnow and 3dnowext feature flags are indeed present. If they are
not, it fallbacks on the most generic x86 kernel. This prevents crashes due to
illegal instruction on qemu guests with a weird configuration.

Closes #272