Don't accept some invalid preprocessor code

Whitespace or non-numeric characters are required after a preprocessor
directive that requires an expression.

CHANGES.current

@@ -7,6 +7,31 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.0.0 (in progress)
 ===========================
 
+2018-01-16: wsfulton
+            Expressions following a preprocessor directive must now be separated by whitespace
+            or non-numeric characters. This syntax change makes the SWIG preprocessor work like
+            the C preprocessor in this area.
+
+            For example, the following code used be accepted as valid syntax:
+              #if1
+              #define ABC 123
+              #endif
+
+            Now you get an error:
+              example.h:1: Error: Unknown SWIG preprocessor directive: if1 (if this is a block of
+              target language code, delimit it with %{ and %})
+              example.h:3: Error: Extraneous #endif.
+
+            The following is the correct syntax:
+              #if 1
+              #define ABC 123
+              #endif
+
+            The following of course also works:
+              #if(1)
+              #define ABC 123
+              #endif
+
 2018-01-15: wsfulton
             Fix issue #1183. Floating point exception evaluating preprocessor expressions
             resulting in division by zero.

Examples/test-suite/errors/pp_expressions_bad.i

@@ -24,3 +24,20 @@
 #if 0
 #elif 8.8
 #endif
+
+/* Missing whitespace after preproc directive */
+#if123
+#endif
+
+#if456e
+#endif
+
+#if 0
+#warning This should not warn
+#elif1
+#warning This should also not warn
+#endif
+
+#if(1)
+#warning Warning okay: #if(1)
+#endif

Examples/test-suite/errors/pp_expressions_bad.stderr

@@ -12,3 +12,8 @@ pp_expressions_bad.i:21: Warning 202: Could not evaluate expression '2e3'
 pp_expressions_bad.i:21: Warning 202: Error: 'Floating point constant in preprocessor expression'
 pp_expressions_bad.i:25: Warning 202: Could not evaluate expression '8.8'
 pp_expressions_bad.i:25: Warning 202: Error: 'Floating point constant in preprocessor expression'
+pp_expressions_bad.i:29: Error: Unknown SWIG preprocessor directive: if123 (if this is a block of target language code, delimit it with %{ and %})
+pp_expressions_bad.i:30: Error: Extraneous #endif.
+pp_expressions_bad.i:32: Error: Unknown SWIG preprocessor directive: if456e (if this is a block of target language code, delimit it with %{ and %})
+pp_expressions_bad.i:33: Error: Extraneous #endif.
+pp_expressions_bad.i:42: Warning 204: CPP #warning, "Warning okay: #if(1)".

Source/Preprocessor/cpp.c

@@ -1454,7 +1454,7 @@ String *Preprocessor_parse(String *s) {
       break;
 
     case 41:			/* Build up the name of the preprocessor directive */
-      if ((isspace(c) || (!isalpha(c)))) {
+      if ((isspace(c) || (!isidchar(c)))) {
 	Clear(value);
 	Clear(comment);
 	if (c == '\n') {