Enable compilation on FreeBSD + clang.

[cbrueffer]

No description provided.

[junaruga]

I think this part is useful. Not only when building by clang but also building by gcc-N compiler.

[ch4rr0]

I thought I removed the g++ defaults in bug_fixes. I may have forgotten to commit the changes.

[junaruga]

@ch4rr0 You are right. I missed the change in master. g++ defaults are not set in Makefile in master.
This repo's Travis has both gcc-N and clang jobs, and it works as expected.

The logic deciding BITS is like this.

bowtie/Makefile

Lines 150 to 152 in 1cc0a45

	ifeq (1,$(shell echo __LP64__ | $(CC) -P -E - | tr -d '\n'))
	BITS=64
	endif

@cbrueffer , maybe you can run the Makefile on FreeBSD without patching, and you can close this PR.