MINOR: tools: add an unsetenv() implementation

Older Solaris and AIX versions do not have unsetenv(). This adds a
fairly simple implementation which scans the environment, for use
with those systems. It will simply require to pass the define in
the "DEFINE" macro at build time like this :

      DEFINE="-Dunsetenv=my_unsetenv"

include/common/standard.h

@@ -1224,6 +1224,16 @@ char *memprintf(char **out, const char *format, ...)
  */
 char *indent_msg(char **out, int level);
 
+/* removes environment variable <name> from the environment as found in
+ * environ. This is only provided as an alternative for systems without
+ * unsetenv() (old Solaris and AIX versions). THIS IS NOT THREAD SAFE.
+ * The principle is to scan environ for each occurence of variable name
+ * <name> and to replace the matching pointers with the last pointer of
+ * the array (since variables are not ordered).
+ * It always returns 0 (success).
+ */
+int my_unsetenv(const char *name);
+
 /* Convert occurrences of environment variables in the input string to their
  * corresponding value. A variable is identified as a series of alphanumeric
  * characters or underscores following a '$' sign. The <in> string must be

src/standard.c

@@ -3699,6 +3699,38 @@ char *indent_msg(char **out, int level)
 	return ret;
 }
 
+/* removes environment variable <name> from the environment as found in
+ * environ. This is only provided as an alternative for systems without
+ * unsetenv() (old Solaris and AIX versions). THIS IS NOT THREAD SAFE.
+ * The principle is to scan environ for each occurence of variable name
+ * <name> and to replace the matching pointers with the last pointer of
+ * the array (since variables are not ordered).
+ * It always returns 0 (success).
+ */
+int my_unsetenv(const char *name)
+{
+	extern char **environ;
+	char **p = environ;
+	int vars;
+	int next;
+	int len;
+
+	len = strlen(name);
+	for (vars = 0; p[vars]; vars++)
+		;
+	next = 0;
+	while (next < vars) {
+		if (strncmp(p[next], name, len) != 0 || p[next][len] != '=') {
+			next++;
+			continue;
+		}
+		if (next < vars - 1)
+			p[next] = p[vars - 1];
+		p[--vars] = NULL;
+	}
+	return 0;
+}
+
 /* Convert occurrences of environment variables in the input string to their
  * corresponding value. A variable is identified as a series of alphanumeric
  * characters or underscores following a '$' sign. The <in> string must be