Add compat/setenv.c, use in git.c.

There is no setenv() in Solaris 5.8.  The trivial calls to
setenv() were replaced by putenv() in a much earlier patch,
but setenv() was used again in git.c.  This patch just adds
a compat/setenv.c.

The rule for building git$(X) also needs to include compat.
objects and compiler flags.  Those are now in makefile vars
COMPAT_OBJS and COMPAT_CFLAGS.

Signed-off-by: E. Jason Riedy <ejr@cs.berkeley.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>

compat/setenv.c

@@ -0,0 +1,31 @@
+#include <stdlib.h>
+#include <string.h>
+
+int gitsetenv(const char *name, const char *value, int replace)
+{
+	int out;
+	size_t namelen, valuelen;
+	char *envstr;
+
+	if (!name || !value) return -1;
+	if (!replace) {
+		char *oldval = NULL;
+		oldval = getenv(name);
+		if (oldval) return 0;
+	}
+
+	namelen = strlen(name);
+	valuelen = strlen(value);
+	envstr = malloc((namelen + valuelen + 2) * sizeof(char));
+	if (!envstr) return -1;
+
+	memcpy(envstr, name, namelen);
+	envstr[namelen] = '=';
+	memcpy(envstr + namelen + 1, value, valuelen);
+	envstr[namelen + valuelen + 1] = 0;
+
+	out = putenv(envstr);
+
+	free(envstr);
+	return out;
+}