The value of SYSHOST=Plan9 was not correctly changed to SYSHOST=Linux in mkconfig.
Using the default value I get the same error:
$ ./makemk.sh
removing old libraries and binaries
gcc -m32 -c -I/tmp/testdir/inferno/Plan9/386/include -I/tmp/testdir/inferno/include -I/tmp/testdir/inferno/utils/include regaux.c regcomp.c regerror.c regexec.c regsub.c rregexec.c rregsub.c
In file included from /tmp/testdir/inferno/Plan9/386/include/../../include/lib9.h:1,
from /tmp/testdir/inferno/Plan9/386/include/lib9.h:1,
from regaux.c:1:
/tmp/testdir/inferno/Plan9/386/include/u.h:1:10: fatal error: /386/include/u.h: No such file or directory
#include "/386/include/u.h"
^~~~~~~~~~~~~~~~~~
compilation terminated.
With SYSHOST=Linux it compiles...
$ ./makemk.sh
removing old libraries and binaries
gcc -m32 -c -I/tmp/testdir/inferno/Linux/386/include -I/tmp/testdir/inferno/include -I/tmp/testdir/inferno/utils/include regaux.c regcomp.c regerror.c regexec.c regsub.c rregexec.c rregsub.c
ar crvs /tmp/testdir/inferno/Linux/386/lib/libregexp.a regaux.o regcomp.o regerror.o regexec.o regsub.o rregexec.o rregsub.o
a - regaux.o
a - regcomp.o
a - regerror.o
a - regexec.o
a - regsub.o
a - rregexec.o
a - rregsub.o
...