gpgme 1.8 build failure

[Robert J. Hansen]

> Sorry, I don't understand the problem. In line 35 keygenerationresult.cpp 
> includes <string.h> so strdup should be available. Is string.h not the right 
> header for strdup on macOS?

It is, but it's in an #ifdef guard.  A small test program is able to use
strdup:

=====
quorra:~ rjh$ more test.cxx
#include <cstring>

int main()
{
  const char* foo = "Foo";
  const char* bar = strdup(foo);
  return 0;
}

quorra:~ rjh$ clang++ -W -Wextra -std=c++11 test.cxx -o t
=====

So given that a dummy program can see strdup, but a default ./configure
gpgme-1.8.0 can't, I suspect somewhere the configure script is setting a
custom #define which is causing the build to fail.