Import bmake-20180222

From ChangeLog

• VERSION: 20180222 Merge with NetBSD make, pick up o parse.c: avoid calling sysconf for every call to loadfile

• VERSION: 20180218 Merge with NetBSD make, pick up o var.c: Var_Set handle NULL value anytime.

• VERSION: 20180212 Merge with NetBSD make, pick up o parse.c: do not treat .info as warning with -W

• VERSION: 20171207 Merge with NetBSD make, pick up o var.c: Var_Append use Var_Set if var not previously set so that VAR_CMD is handled correctly. Add a suitable unit-test.

• VERSION: 20171126

• aclocal.m4: use AC_LINK_IFELSE for AC_C___ATTRIBUTE__ since AC_TRY_COMPILE puts input inside main() which upsets modern compilers.

• VERSION: 20171118 Merge with NetBSD make, pick up o var.c: do not append to variable set on command line add unit-test to catch this.