Use libnettle rather than libcrypto/openssl for SHA1

Reason:
https://github.com/ValveSoftware/steam-for-linux/issues/4504

mesa.spec

@@ -31,7 +31,7 @@
 Summary:	OpenGL-compatible 3D graphics library
 Name:		mesa
 Version:	13.0.2
-Release:	3
+Release:	4
 License:	MIT
 Group:		System/Libraries
 Url:		http://www.mesa3d.org
@@ -86,8 +86,10 @@ BuildRequires:	libselinux-devel
 %endif
 %if %{with vulkan}
 BuildRequires:	vulkan-devel
-# for SHA1 implementation from libcrypto:
-BuildRequires:	pkgconfig(openssl)
+# for SHA1 implementation from libnettle:
+BuildRequires:	pkgconfig(nettle)
+# [NB] Why not libcrypto & openssl? That is why:
+# https://github.com/ValveSoftware/steam-for-linux/issues/4504
 %endif
 
 # [NB] Do not mark it noarch, otherwise rpm will be confused on where 
@@ -686,7 +688,7 @@ autoreconf -vfi
 	--enable-texture-float=yes \
 %if %{with vulkan}
 	%{vulkan_drivers} \
-	--with-sha1=libcrypto \
+	--with-sha1=libnettle \
 %endif
 	--enable-xvmc