diff --git a/Makefile b/Makefile
index a566bd9aed..13aa24f918 100644
--- a/Makefile
+++ b/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.
diff --git a/config.mak.in b/config.mak.in
index 15fb26cb92..40b14d985a 100644
--- a/config.mak.in
+++ b/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@
diff --git a/configure.ac b/configure.ac
index 6f641e32f0..af177fdb4d 100644
--- a/configure.ac
+++ b/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=],