165994 – [patch] fix several usages of ${SRC_BASE} before it is defined

no flags

SRC_BASE is being used before being defined in several ports. it happened to 
work anyways in most cases because the main use of SRC_BASE is for checking if
${SRC_BASE}/sys/ (/usr/src/sys/) exists, and if SRC_BASE is empty, it just
checks for /sys/ which does exist on most systems as a symlink to /usr/src/sys/.

example from sysutils/graid5 (note graid5 has been independently fixed since
this was made):

.if !exists(${SRC_BASE}/sys/Makefile)
IGNORE= requires kernel source files
.elif !exists(${SRC_BASE}/sbin/geom/Makefile)
IGNORE= requires sbin source files
.endif

===>  graid5-1.1.20110927.40 requires sbin source files.
*** Error code 1

65254: stat("/sys/Makefile",{ mode=-rw-r--r-- ,inode=781342,size=1677,blksize=4096 }) = 0 (0x0)
65254: stat("/sbin/geom/Makefile",0x7fffffffd5d0) ERR#20 'Not a directory'

the ${SRC_BASE}/sys/Makefile happens to work due to the /sys symlink.
switch to bsd.port.pre/post.mk and files are correctly checked.

73664: stat("/usr/src/sys/Makefile",{ mode=-rw-r--r-- ,inode=781342,size=1677,blksize=4096 }) = 0 (0x0)
73664: stat("/usr/src/sbin/geom/Makefile",{ mode=-rw-r--r-- ,inode=685289,size=525,blksize=4096 }) = 0 (0x0)

attached patch fixes this in several ports in different ways, due to differing
Makefile usages and structures.

Fix: Patch attached with submission follows: