Configure: call "make clean" for OpenSSL only if Makefile exists.

This change allows to build nginx against git checkout of OpenSSL.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

auto/lib/openssl/make

diff --git a/auto/lib/openssl/make b/auto/lib/openssl/make
index 9090fcb..e64acd9 100644
--- a/auto/lib/openssl/make
+++ b/auto/lib/openssl/make
@@ -55,7 +55,7 @@
 
 $OPENSSL/.openssl/include/openssl/ssl.h:	$NGX_MAKEFILE
 	cd $OPENSSL \\
-	&& \$(MAKE) clean \\
+	&& if [ -f Makefile ]; then \$(MAKE) clean; fi \\
 	&& ./config --prefix=$ngx_prefix no-shared $OPENSSL_OPT \\
 	&& \$(MAKE) \\
 	&& \$(MAKE) install LIBDIR=lib