make: refuse empty variable names

Found by accident while looking at www/dufs/crates.inc

gmake refuses empty variable names, I think we should too.

Especially if we make typos and end up expanding empty stuff.

(currently, stuff that starts with an = will be treated as a comment)

This needs to get through a full release, obviously...

Index: usr.bin/make/parsevar.c
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.bin/make/parsevar.c,v
diff -u -p -r1.18 parsevar.c
--- usr.bin/make/parsevar.c	18 Jun 2024 02:11:03 -0000	1.18
+++ usr.bin/make/parsevar.c	14 Jun 2026 08:54:16 -0000
@@ -72,6 +72,9 @@ parse_variable_assignment(const char *li
 
 	arg = VarName_Get(line, &name, NULL, true, find_op1);
 
+	if (name.s == name.e)
+		Parse_Error(PARSE_FATAL, "empty variable name");
+
 	while (ISSPACE(*arg))
 		arg++;
 
Index: regress/usr.bin/make/Makefile
===================================================================
RCS file: /build/data/openbsd/cvs/src/regress/usr.bin/make/Makefile,v
diff -u -p -r1.51 Makefile
--- regress/usr.bin/make/Makefile	2 Sep 2021 07:14:15 -0000	1.51
+++ regress/usr.bin/make/Makefile	14 Jun 2026 09:00:21 -0000
@@ -4,7 +4,7 @@ REGRESS_TARGETS=     t1  t2  t3  t4  t5 
 		t10 t11 t12 t13 t14 t15 t16 t17 t18 t19 \
 		t20 t21 t22 t23 t24 t25 t26 t27 t28 t29 \
 		t30 t31 t32 t33 t34 t35 t36 t37 t38 t38j t39 \
-		t40 t41 t42 t42bis t43 t44 t45 t46 t47 t48
+		t40 t41 t42 t42bis t43 t44 t45 t46 t47 t48 t49
 
 REGRESS_EXPECTED_FAILURES = t14 t17 t18
 
@@ -204,6 +204,11 @@ t47: tok.o
 
 t48:
 	cd ${.CURDIR} && make -r -f mk43 a
+
+t49:
+	if cd ${.CURDIR} && env -i PATH=${PATH} ${MAKE} -r -f mk44 all 2>/dev/null; then \
+	    false; \
+	fi
 
 t1.out:
 	echo MACHINE_ARCH=${MACHINE_ARCH} >$@
Index: regress/usr.bin/make/mk44
===================================================================
RCS file: regress/usr.bin/make/mk44
diff -N regress/usr.bin/make/mk44
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ regress/usr.bin/make/mk44	14 Jun 2026 08:58:01 -0000
@@ -0,0 +1,7 @@
+# empty variable !
+=
+
+all:
+	echo "not okay"
+
+.PHONY: all