Makefile.am: avoid double `bison` execution on parallel build
Before the change `make -j16` ran `bison` twice on the same input:
$ make -j16
...
make[1]: Entering directory '/build/source/Source'
bison -d -Wall -Werror --output=CParse/parser.c ./CParse/parser.y
gcc -g -O2 -Wall -W -I. -c -o mdfour.o mdfour.c
bison -d -Wall -Werror --output=CParse/parser.c ./CParse/parser.y
...
Note how two parallel `bison` calls both generate the same files. It works
most of the time, but is prone to generate unexpected output.
This happens because Makefile rules like:
a b: c; COMMAND
all: a b
will run COMMAND for each target (twice here: once for `a` and once for
`b`),
The fix uses GNU make's Grouped Targets to use single command to produce
multiple targets.
diff --git a/Source/Makefile.am b/Source/Makefile.am
index 9a1d241..b9bc13b 100644
--- a/Source/Makefile.am
+++ b/Source/Makefile.am
@@ -99,7 +99,9 @@
bin_PROGRAMS = eswig
-CParse/parser.c CParse/parser.h: CParse/parser.y
+# Use GNU make's grouped targets to avoid repeated bison execution for
+# each target.
+CParse/parser.c CParse/parser.h &: CParse/parser.y
$(BISON) $(AM_YFLAGS) $(YFLAGS) --output=CParse/parser.c $(srcdir)/CParse/parser.y
# The executable is copied to the root directory for installation and running the test-suite.
