Solve occasional poudriere build failures by adding MAKE_JOBS_UNSAFE.

Sample poudriere build error without MAKE_JOBS_UNSAFE:

    Error: Could not find class file for 'gnu.io.Raw'.
    gmake[1]: *** [Makefile:613: /wrkdirs/usr/ports/comms/rxtx/work/rxtx-2.2pre2/gnu/io/NoSuchPortException.class] Error 1

PR:		216558
Reviewed by:	mat, matthew (mentor)
Approved by:	mat, koobs (maintainer), matthew (mentor)
Differential Revision:	https://reviews.freebsd.org/D18999

comms/rxtx/Makefile

@@ -28,6 +28,7 @@ CONFIGURE_ENV=	JAVA_HOME=${JAVA_HOME}
 INSTALL_TARGET=	install-strip
 
 USE_LDCONFIG=	yes
+MAKE_JOBS_UNSAFE=	yes
 
 PLIST_SUB=	JAVA_HOME=${JAVA_HOME:S,^${PREFIX}/,,}
 PLIST_SUB+=	ARCH=${ARCH:C/armv.*/arm/:S/powerpc64/ppc64/}