acinclude.m4: fix ./configure --with-libgcrypt

The change fixes passing of bogus gcrypt prefix.
Reproducible as:

    $ ./configure --with-libgcrypt
    $ make V=1
    ...
    /bin/sh ../libtool  --tag=CC   --mode=link gcc  -g -O2 -Iyes/include -version-info 1:1:0 -no-undefined -export-symbols-regex '^libssh2_.*' -lgcrypt  -lz -Lyes/lib -o libssh2.la -rpath /usr/local/lib channel.lo comp.lo crypt.lo hostkey.lo kex.lo mac.lo misc.lo packet.lo publickey.lo scp.lo session.lo sftp.lo userauth.lo transport.lo version.lo knownhost.lo agent.lo libgcrypt.lo pem.lo keepalive.lo global.lo   -lgcrypt
    ../libtool: line 7475: cd: yes/lib: No such file or directory
    libtool:   error: cannot determine absolute directory name of 'yes/lib'

These
    -Iyes/include
    -Lyes/lib
come from libgcrypt code autodetection:
  if test -n "$use_libgcrypt" && test "$use_libgcrypt" != "no"; then
     LDFLAGS="$LDFLAGS -L$use_libgcrypt/lib"
     CFLAGS="$CFLAGS -I$use_libgcrypt/include"

I assume it's a typo to use yes/no flag as a prefix and changed
it to '$with_libgcrypt_prefix'.

Reported-by: Mikhail Pukhlikov <cynede@gentoo.org>
Signed-off-by: Sergei Trofimovich <siarheit@google.com>

acinclude.m4

@@ -412,9 +412,9 @@ AC_DEFUN([LIBSSH2_CHECKFOR_GCRYPT], [
 
   old_LDFLAGS=$LDFLAGS
   old_CFLAGS=$CFLAGS
-  if test -n "$use_libgcrypt" && test "$use_libgcrypt" != "no"; then
-    LDFLAGS="$LDFLAGS -L$use_libgcrypt/lib"
-    CFLAGS="$CFLAGS -I$use_libgcrypt/include"
+  if test -n "$with_libgcrypt_prefix" && test "$use_libgcrypt" != "no"; then
+    LDFLAGS="$LDFLAGS -L$with_libgcrypt_prefix/lib"
+    CFLAGS="$CFLAGS -I$with_libgcrypt_prefix/include"
   fi
   AC_LIB_HAVE_LINKFLAGS([gcrypt], [], [
     #include <gcrypt.h>