Reduce a little bit foot shooting for people installing python 3 ports using

PYTHON_ABIVER/PYTHON_INCLUDEDIR, when they use default options for
lang/python3x, by providing a reasonable default value to PYTHON_ABIVER

With hat:	portmgr

Mk/Uses/python.mk

@@ -407,8 +407,13 @@ PYTHON_REL=	${PYTHON_PORTVERSION:C/^([0-9]+\.[0-9]+\.[0-9]+).*/\1/:C/\.([0-9]+)$
 
 # Might be overridden by calling ports
 PYTHON_CMD?=		${_PYTHON_BASECMD}${_PYTHON_VERSION}
-.if exists(${PYTHON_CMD}-config) && ${PYTHON_VER} != 2.7
+.if ${PYTHON_VER} != 2.7
+.if exists(${PYTHON_CMD}-config)
 PYTHON_ABIVER!=		${PYTHON_CMD}-config --abiflags
+.else
+# Default ABI flags for lang/python3x ports
+PYTHON_ABIVER=		m
+.endif
 .endif
 
 .if !defined(PYTHONBASE)