libssh2: fix library search path issues

Fix issues with some toolchains not finding libssl or libgcrypt.

Fixes http://autobuild.buildroot.net/results/eeaaf41c0c5f88357af8445750dce776774c2b5d
Fixes http://autobuild.buildroot.net/results/7a2590a7c3a1f52590f3e886b7cf9d04ad966a10

[Peter: indentation fixup]
Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libssh2/libssh2.mk

@@ -15,10 +15,14 @@ LIBSSH2_CONF_OPT = --disable-examples-build
 # Only select openssl if libgcrypt is not selected
 ifeq ($(BR2_PACKAGE_LIBGCRYPT),y)
 LIBSSH2_DEPENDENCIES += libgcrypt
-LIBSSH2_CONF_OPT += --with-libgcrypt --without-openssl
+LIBSSH2_CONF_OPT += --with-libgcrypt \
+	--with-libgcrypt-prefix=$(STAGING_DIR)/usr \
+	--without-openssl
 else
 LIBSSH2_DEPENDENCIES += openssl
-LIBSSH2_CONF_OPT += --with-openssl --without-libgcrypt
+LIBSSH2_CONF_OPT += --with-openssl \
+	--with-libssl-prefix=$(STAGING_DIR)/usr \
+	--without-libgcrypt
 endif
 
 # Add zlib support if enabled