scons: Avoid malloc/free compiler optimization when using tcmalloc

scons: Avoid malloc/free compiler optimization when using tcmalloc

According to the tcmalloc readme, the recommended way of compiling
applications that make use of tcmalloc is to disable compiler
optimizations that make assumptions about malloc and friends. This
changeset adds the necessary compiler flags for both gcc and clang.

From the tcmalloc readme:
"NOTE: When compiling with programs with gcc, that you plan to link
with libtcmalloc, it's safest to pass in the flags

 -fno-builtin-malloc -fno-builtin-calloc
 -fno-builtin-realloc -fno-builtin-free

when compiling."