huc: main.c: use assembler specified in environment variable PCE_PCEAS

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
test: mk: use PCE_PCEAS to allow tests to run without installing first

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
huc: accept C files with a capital C

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
huc: io.c: print error message on invalid file type

Exiting silently is less than helpful...

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>
huc: setvdc(), scroll(): fix stack pointer arithmetic with -msmall

Small stack code assumes the high byte of the stack pointer is fixed at
0x3f. These functions incorrectly used 16-bit arithmetic to clean their
arguments from the stack, causing the high byte to increment to 0x40 if
the stack is empty afterwards.

Signed-off-by: Ulrich Hecht <ulrich.hecht@gmail.com>