Implement #if and friends.

With this change, tests 41 through 49 all pass. (The defined operator
appears to be somehow broken so that test 50 doesn't pass yet.)

glcpp.h

@@ -49,7 +49,7 @@ typedef struct token_list token_list_t;
 
 typedef union YYSTYPE
 {
-	int ival;
+	intmax_t ival;
 	char *str;
 	string_list_t *string_list;
 	token_t *token;

tests/049-if-expression-precedence.c

@@ -3,4 +3,3 @@ failure with operator precedence
 #else
 success
 #endif
-

tests/050-if-defined.c

@@ -15,5 +15,3 @@ failure_3
 #else
 success_3
 #endif
-
-

tests/glcpp-test

@@ -6,5 +6,5 @@ for test in *.c; do
     ../glcpp < $test > $test.out
     gcc -E $test -o $test.gcc
     grep -v '^#' < $test.gcc > $test.expected
-    diff -u $test.expected $test.out
+    diff -B -u $test.expected $test.out
 done