fix OpenSSL build failure with perl 5.18 for mingw-w64 targets

Install only the software and not the docs because the man page
generation fails with "Expected text after =item, not a number"
due to a breaking change in POD::Simple present in perl 5.18,
which is the case on Ubuntu Trusty.

scripts/build.py

@@ -40,13 +40,13 @@ OPENSSL = {
         },
         'mingw-w64-cross-win32': {
             'configure' : '--cross-compile-prefix=i686-w64-mingw32- no-shared no-asm mingw64',
-            'build'     : ['make', 'make install'],
+            'build'     : ['make', 'make install_sw'],
             'libs'      : ['libssl.a', 'libcrypto.a'],
             'os_libs'   : '-lws2_32 -lgdi32 -lcrypt32'
         },
         'mingw-w64-cross-win64': {
             'configure' : '--cross-compile-prefix=x86_64-w64-mingw32- no-shared no-asm mingw64',
-            'build'     : ['make', 'make install'],
+            'build'     : ['make', 'make install_sw'],
             'libs'      : ['libssl.a', 'libcrypto.a'],
             'os_libs'   : '-lws2_32 -lgdi32 -lcrypt32'
         }