Build errors in a_md5encrypt.c

[csBlueChip]

This may be related to OpenSSL version compatibility issues
https://stackoverflow.com/questions/42662733/evp-md-ctx-error-storage-size-of-ctx-isn-t-known

make  all-am
make[1]: Entering directory '/path-to/src/ntp/libntp'
  CC       a_md5encrypt.o
a_md5encrypt.c: In function 'MD5authencrypt':
a_md5encrypt.c:29:13: error: storage size of 'ctx' isn't known
  EVP_MD_CTX ctx;
             ^~~
a_md5encrypt.c:29:13: warning: unused variable 'ctx' [-Wunused-variable]
a_md5encrypt.c: In function 'MD5authdecrypt':
a_md5encrypt.c:70:13: error: storage size of 'ctx' isn't known
  EVP_MD_CTX ctx;
             ^~~
a_md5encrypt.c:70:13: warning: unused variable 'ctx' [-Wunused-variable]
a_md5encrypt.c: In function 'addr2refid':
a_md5encrypt.c:109:13: error: storage size of 'ctx' isn't known
  EVP_MD_CTX ctx;
             ^~~
a_md5encrypt.c:109:13: warning: unused variable 'ctx' [-Wunused-variable]
Makefile:874: recipe for target 'a_md5encrypt.o' failed
make[1]: *** [a_md5encrypt.o] Error 1
make[1]: Leaving directory '/path-to/src/ntp/libntp'
Makefile:717: recipe for target 'all' failed
make: *** [all] Error 2