Changed drastically because it didn't work correctly:

- Don't cache the --with-openssl-* option arguments
- Only search for openssl/*.h, the openssl include files include other
  files only from an openssl/ directory too
- Set CPPFLAGS before AC_CHECK_HEADERS

m4/openssl.m4

@@ -1,80 +1,31 @@
 dnl Check to find the OpenSSL headers/libraries
 
-
 AC_DEFUN(tinc_OPENSSL,
 [
-  AC_ARG_WITH(openssl,
-    [  --with-openssl=DIR      OpenSSL library and headers prefix],
-    [openssl_lib="$withval/lib"
-     openssl_include="$withval/include"]
+  tinc_ac_save_CPPFLAGS="$CPPFLAGS"
+
+  AC_ARG_WITH(openssl-include,
+    [  --with-openssl-include=DIR  OpenSSL headers directory (without trailing /openssl)],
+    [openssl_include="$withval"
+     CFLAGS="$CFLAGS -I$withval"
+     CPPFLAGS="$CPPFLAGS -I$withval"]
   )
-  AC_CACHE_CHECK([for openssl headers], tinc_cv_openssl_include,
-  [
-    AC_ARG_WITH(openssl-include,
-      [  --with-openssl-include=DIR OpenSSL headers directory],
-      [tinc_cv_openssl_include="$withval"],
-      [if test "x$openssl_include" = "x" ; then
-         tinc_cv_openssl_include="none given"
-       else
-         tinc_cv_openssl_include=$openssl_include
-       fi]
-    )
-  ])
-  AC_CACHE_CHECK([for openssl libraries], tinc_cv_openssl_lib,
-  [
-    AC_ARG_WITH(openssl-lib,
-      [  --with-openssl-lib=DIR  OpenSSL library directory],
-      [tinc_cv_openssl_lib="$withval"],
-      [if test "x$openssl_lib" = "x" ; then
-         tinc_cv_openssl_lib="none given"
-       else
-         tinc_cv_openssl_lib=$openssl_lib
-       fi]
-    )
-  ])
 
-if test "$tinc_cv_openssl_lib" != "none given" ; then
-  LIBS="$LIBS -L$tinc_cv_openssl_lib"
-fi
-if test "$tinc_cv_openssl_include" != "none given" ; then
-  CFLAGS="$CFLAGS -I$tinc_cv_openssl_include"
-fi
+  AC_ARG_WITH(openssl-lib,
+    [  --with-openssl-lib=DIR  OpenSSL library directory],
+    [openssl_lib="$withval"
+     LIBS="$LIBS -L$withval"]
+  )
 
-osi=found
-AC_CHECK_HEADERS(evp.h rsa.h rand.h err.h sha.h pem.h,
-[], [osi=none; break])
-
-if test "$osi" = "none" ; then
-  osi=found
   AC_CHECK_HEADERS(openssl/evp.h openssl/rsa.h openssl/rand.h openssl/err.h openssl/sha.h openssl/pem.h,
-  [], [osi=none; break])
-fi
+    [],
+    [AC_MSG_ERROR("OpenSSL header files not found."); break]
+  )
 
+  CPPFLAGS="$tinc_ac_save_CPPFLAGS"
 
-libcrypto=none
-
-if test "$osi" = "found" ; then
-  AC_CHECK_LIB(crypto, SHA1_version, [
-    libcrypto=yes
-  ])
-fi
-
-if test $libcrypto = none; then
-cat <<EOF
-
-It seems like OpenSSL is not installed on this system.  But perhaps
-you need to supply the exact location of the headers and libraries.
-You can try running configure with the --with-openssl=/DIRECTORY
-parameter.  If you installed the headers and libraries in a different
-location you can use --with-openssl-include=/DIR and
---with-openssl-lib=/DIR.
-
-EOF
-
-  AC_MSG_ERROR(OpenSSL not found.)
-
-else
-  LIBS="$LIBS -lcrypto"
-fi
-
+  AC_CHECK_LIB(crypto, SHA1_version,
+    [LIBS="$LIBS -lcrypto"],
+    [AC_MSG_ERROR("OpenSSL libraries not found.")]
+  )
 ])