Fix mempcpy undefined error under macOs

[haoch]

[ 25%] Linking C executable example_https_server
Undefined symbols for architecture x86_64:
  "_mempcpy", referenced from:
      _htp__evbuffer_add_iovec_ in libevhtp.a(evhtp.c.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[aflin]

Thanks for finding this.
Unless you are ignoring the return value of mempcpy, it is not equivalent to memcpy. See the linux man page. In evhtp.c, it appears the return value of mempcpy is being used in a loop to append a buffer. Perhaps instead something like this (untested)?

#ifndef mempcpy
#define mempcpy(d,s,n)  memcpy((d),(s),(n)) + (n) 
#endif

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