[SV 59230] Ensure environment variables are exportable

When checking for invalid environment variable names we searched the
entire name string instead of just the first LENGTH chars; this could
cause us to incorrectly decide the variable was not exportable.

Dmitry Goncharov <dgoncharov@users.sf.net> found this bug and
provided a test case and sample fix: I used the test but chose a
slightly different fix.

* src/variable.c (define_variable_in_set): check the variable name
not the input string.
* tests/scripts/features/targetvars: Ensure environment variable
values are exported.

tests/scripts/features/targetvars

@@ -281,6 +281,16 @@ all: ; $sname
 unlink("sd/$sname");
 rmdir ('sd');
 
+# SV 59230: Conditional (non-)assignment of target-specific variables should
+# preserve export settings.
+
+$ENV{hello} = 'moon';
+run_make_test(q!
+all:; @echo hello=$$hello
+dummy: hello?=world
+!,
+              '', 'hello=moon');
+
 # TEST #19: Test define/endef variables as target-specific vars
 
 # run_make_test('