Making tcmalloc the default when compatible with host compiler
Using target compiler instead of host compiler
Simplifying logic
Not using tcmalloc on cygwin, for now
Merge pull request #320 from benharsh/tcmalloc

This small script change makes tcmalloc the default allocator for compatible compilers: gnu, intel, and clang.

For now, tcmalloc will not be the default for cygwin.