Using make's `-j` breaks the build

[jendrikw]

Originally found by a user of the AUR package: https://aur.archlinux.org/packages/agbcc

This is an example of the output when I run make -C gcc -j 16:

make: Entering directory '/tmp/agbcc/gcc'
gcc -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c gengenrtl.c
gcc -MT rtl.o -MMD -MP -MF .d/rtl.Td -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c rtl.c
gcc -MT bitmap.o -MMD -MP -MF .d/bitmap.Td -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c bitmap.c
gcc -MT obstack.o -MMD -MP -MF .d/obstack.Td -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c obstack.c
gcc -MT print-rtl.o -MMD -MP -MF .d/print-rtl.Td -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c print-rtl.c
gcc -MT rtlanal.o -MMD -MP -MF .d/rtlanal.Td -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c rtlanal.c
gcc -g -std=gnu11 -Werror-implicit-function-declaration -Wno-error=incompatible-pointer-types  -I. -I. -c gencheck.c
In file included from rtl.c:24:
rtl.h:1116:10: fatal error: genrtl.h: No such file or directory
 1116 | #include "genrtl.h"
      |          ^~~~~~~~~~
compilation terminated.
make: *** [Makefile:85: rtl.o] Error 1
make: *** Waiting for unfinished jobs....
In file included from rtlanal.c:24:
rtl.h:1116:10: fatal error: genrtl.h: No such file or directory
 1116 | #include "genrtl.h"
      |          ^~~~~~~~~~
compilation terminated.
make: *** [Makefile:85: rtlanal.o] Error 1
In file included from bitmap.c:23:
rtl.h:1116:10: fatal error: genrtl.h: No such file or directory
 1116 | #include "genrtl.h"
      |          ^~~~~~~~~~
compilation terminated.
make: *** [Makefile:85: bitmap.o] Error 1
In file included from print-rtl.c:24:
rtl.h:1116:10: fatal error: genrtl.h: No such file or directory
 1116 | #include "genrtl.h"
      |          ^~~~~~~~~~
compilation terminated.
make: *** [Makefile:85: print-rtl.o] Error 1
mv -f .d/obstack.Td .d/obstack.d
make: Leaving directory '/tmp/agbcc/gcc'

I believe the problem is just incomplete dependency specifications in the makefile, probably because the DEPFLAGS thing is run only once and would have to account for some c files that are generated.

A workaround would be to use the special .NOTPARALLEL target (https://www.gnu.org/software/make/manual/html_node/Parallel-Disable.html).

[mid-kid]

I encountered this myself since I have $MAKEFLAGS in my environment, I added -j1 to the build.sh, nobody's maintaining the codebase, so consider doing the same.

Worked around in da598c1