conditionally build certhash into openssl(1)

For now, look for openat and symlink. We may switch to just needing
symlink later.

apps/Makefile.am

@@ -10,7 +10,6 @@ openssl_LDADD += $(top_builddir)/crypto/libcrypto.la
 openssl_SOURCES = apps.c
 openssl_SOURCES += asn1pars.c
 openssl_SOURCES += ca.c
-openssl_SOURCES += certhash.c
 openssl_SOURCES += ciphers.c
 openssl_SOURCES += cms.c
 openssl_SOURCES += crl.c
@@ -58,6 +57,12 @@ openssl_SOURCES += verify.c
 openssl_SOURCES += version.c
 openssl_SOURCES += x509.c
 
+if BUILD_CERTHASH
+openssl_SOURCES += certhash.c
+else
+openssl_SOURCES += certhash_disabled.c
+endif
+
 if HOST_WIN
 openssl_SOURCES += apps_win.c
 else