build-sys: add a rule to print a variable

qemu with hax to log dma reads & writes jcs.org/2018/11/12/vfio

$ make print-CFLAGS
CFLAGS=-fsanitize=address -Og -g

Trick from various sources:
https://stackoverflow.com/questions/16467718/how-to-print-out-a-variable-in-makefile
https://www.cmcrossroads.com/article/printing-value-makefile-variable

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180104160523.22995-4-marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

authored by

Marc-André Lureau and committed by

Paolo Bonzini 8 years ago de1da442 42a77f1c

+17 -1

2 changed files

expand all

Makefile

docs

devel

build-system.txt

+4 -1

Makefile

··· 8 8 9 9 UNCHECKED_GOALS := %clean TAGS cscope ctags dist \ 10 10 html info pdf txt \ 11 - help check-help \ 11 + help check-help print-% \ 12 12 docker docker-% vm-test vm-build-% 13 + 14 + print-%: 15 + @echo '$*=$($*)' 13 16 14 17 # All following code might depend on configuration variables 15 18 ifneq ($(wildcard config-host.mak),)

+13

docs/devel/build-system.txt

··· 510 510 This is the entrypoint used when make recurses to build a single system 511 511 or userspace emulator target. It is merely a symlink back to the 512 512 Makefile.target in the top level. 513 + 514 + 515 + Useful make targets 516 + =================== 517 + 518 + - help 519 + 520 + Print a help message for the most common build targets. 521 + 522 + - print-VAR 523 + 524 + Print the value of the variable VAR. Useful for debugging the build 525 + system.