Rename BSDmakefile to Makefile to appease OpenBSD

OpenBSD dropped support for BSDmakefile and GNU make looks for GNUmakefile first, so rename BSDmakefile -> Makefile.

"By default, when make looks for the makefile, it tries the following names, in order: GNUmakefile, makefile and Makefile." - http://www.gnu.org/software/make/manual/make.html#Makefile-Names