autoconf: Add checking for unsetenv function

Update configure.ac (and config.mak.in) by adding test for unsetenv
(NO_UNSETENV).  Add comment about NO_UNSETENV to Makefile header, as
original commit 731043fd adding compat/unsetenv.c didn't do that.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -38,6 +38,8 @@ all::
 #
 # Define NO_SETENV if you don't have setenv in the C library.
 #
+# Define NO_UNSETENV if you don't have unsetenv in the C library.
+#
 # Define NO_MKDTEMP if you don't have mkdtemp in the C library.
 #
 # Define NO_SYMLINK_HEAD if you never want .git/HEAD to be a symbolic link.

config.mak.in

@@ -40,6 +40,7 @@ NO_MEMMEM=@NO_MEMMEM@
 NO_STRLCPY=@NO_STRLCPY@
 NO_STRTOUMAX=@NO_STRTOUMAX@
 NO_SETENV=@NO_SETENV@
+NO_UNSETENV=@NO_UNSETENV@
 NO_MKDTEMP=@NO_MKDTEMP@
 NO_ICONV=@NO_ICONV@
 OLD_ICONV=@OLD_ICONV@

configure.ac

@@ -356,6 +356,12 @@ AC_CHECK_FUNC(setenv,
 [NO_SETENV=YesPlease])
 AC_SUBST(NO_SETENV)
 #
+# Define NO_UNSETENV if you don't have unsetenv in the C library.
+AC_CHECK_FUNC(unsetenv,
+[NO_UNSETENV=],
+[NO_UNSETENV=YesPlease])
+AC_SUBST(NO_UNSETENV)
+#
 # Define NO_MKDTEMP if you don't have mkdtemp in the C library.
 AC_CHECK_FUNC(mkdtemp,
 [NO_MKDTEMP=],