Add new warnings invalid-var and invalid-ref

The "invalid-var" warning triggers if the makefile attempts to assign
a value to an invalid variable name (a name containing whitespace).
The "invalid-ref" warning triggers if the makefile attempts to
reference an invalid variable name.  Both new warnings have a default
action of "warn".

* NEWS: Add these new warnings.
* doc/make.1: Document them in the man page.
* doc/make.texi (Warnings): Document them in the user's manual.
* src/warning.h: Add enum values for the new warning types.
* src/main.c (initialize_warnings): Initialize the new warnings.
* src/variable.h (undefine_variable_in_set, undefine_variable_global):
Ask callers to provide a struct floc specifying where the variable
is undefined.
* src/read.c (do_undefine): Pass floc when undefining.
* src/variable.c (check_valid_name): If invalid-var is enabled, check
the variable name.
(define_variable_in_set): Call it.
(undefine_variable_in_set): Ditto.
(check_variable_reference): If invalid-ref is enabled, check the
variable reference.
(lookup_variable): Call it.
(lookup_variable_in_set): Ditto.
* tests/scripts/options/warn: Add tests for the new warning types.

2 files changed, 19 insertions, 2 deletions

diff --git a/doc/make.1 b/doc/make.1
index 933a8273..8a407b03 100644
--- a/doc/make.1
+++ b/doc/make.1
@@ -374,6 +374,10 @@ can be an action; one of
 or
 .I error
 to set the default action for all warnings, or it can be a specific warning:
+.I invalid-var
+(assigning to an invalid variable name),
+.I invalid-ref
+(referencing an invalid variable name), or
 .I undefined-var
 (referencing an undefined variable).  The behavior of each warning can be set
 by adding
@@ -387,6 +391,12 @@ is provided the action for all warnings is
 If no
 .B \-\-warn
 option is provided the default action for
+.I invalid-var
+and
+.I invalid-ref
+is
+.I warn
+and the default action for
 .I undefined-var
 is
 .IR ignore .
diff --git a/doc/make.texi b/doc/make.texi
index 31db623b..b3cc57a0 100644
--- a/doc/make.texi
+++ b/doc/make.texi
@@ -9313,6 +9313,12 @@ GNU Make can detect some types of incorrect usage in makefiles and show
 warnings about them.  Currently these issues can be detected:
 
 @table @samp
+@item invalid-var
+Assigning to an invalid variable name (e.g., a name containing whitespace).
+
+@item invalid-ref
+Using an invalid variable name in a variable reference.
+
 @item undefined-var
 Referencing a variable that has not been defined.
 @end table
@@ -9331,8 +9337,9 @@ Show a warning about the usage and continue processing the makefile.
 Show an error for the usage and immediately stop processing the makefile.
 @end table
 
-The default action when no warning control options are provided for
-@samp{undefined-var} is @samp{warn}.
+The default action of GNU Make when no warning control options are provided
+is @samp{ignore} for @samp{undefined-var}, and @samp{warn} for
+@samp{invalid-var} and @samp{invalid-ref}.
 
 To modify this default behavior, you can use the @code{--warn} option.  This
 option can be specified on the command line, or by adding it to the