Fix Appveyor build.

Change the openssl-devel package to libssl-devel.  Cygwin still has an
"obsolete" (essentially virtual) package for openssl-devel, but it
doesn't seem to work properly on Appveyor's Cygwin install.

Protobuf >= 3.6.0 requires C++11, which I added support for last year.
But when I did that, I requested strict ANSI C++ compatibility, which
causes Cygwin/newlib's libc feature test macros to be set to disable
(at least) POSIX.1 extensions.  Let the Autoconf macro use its default
instead (prefer GNU/extended C++, accept anything).

appveyor.yml

@@ -31,8 +31,8 @@ init:
 # Install needed build dependencies
 # 
 install:
-    - '%CYG_ROOT%\setup-x86_64.exe --quiet-mode --no-shortcuts --upgrade-also --packages autoconf,autogen,automake,cygwin-devel,git,gcc-core,gcc-g++,libncurses-devel,libprotobuf-devel,make,openssh,openssl-devel,perl,perl_pods,pkg-config,tmux,zlib-devel'
-    - '%CYG_ROOT%\bin\cygcheck -dc cygwin'
+    - '%CYG_ROOT%\setup-x86_64.exe --quiet-mode --no-shortcuts --upgrade-also --packages autoconf,autogen,automake,cygwin-devel,git,gcc-core,gcc-g++,libncurses-devel,libprotobuf-devel,libssl-devel,make,openssh,perl,perl_pods,pkg-config,tmux,zlib-devel'
+    - '%CYG_ROOT%\bin\cygcheck -dc'
 
 #
 # Mosh uses symlinks and other Unixisms in the work dir.  Fix by replacing it with a Cygwin checkout.

configure.ac

@@ -20,8 +20,9 @@ AS_IF([test x"$PROTOC" = x],
 # automake 1.12 seems to require this, but automake 1.11 doesn't recognize it
 m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 
+# Protobuf 3.6+ requires C++11.
 AS_IF([pkg-config --atleast-version 3.6.0 protobuf],
-  [AX_CXX_COMPILE_STDCXX([11], [noext])])
+  [AX_CXX_COMPILE_STDCXX([11])])
 
 WARNING_CXXFLAGS=""
 PICKY_CXXFLAGS=""