This script finds system OpenSSL despite I want it to use LibreSSL from /opt/libressl
Do not find it automatically -- we have set $LIBS and $PREFIX_OPENSSL
// mikhailnov, 25.12.2019

--- curl-7.67.0/configure.ac.orig	2019-12-25 21:48:24.085383749 +0300
+++ curl-7.67.0/configure.ac	2019-12-25 21:57:06.765662329 +0300
@@ -1642,9 +1642,8 @@
     dnl no --with-ssl option given, just check default places
     if test x$cross_compiling != xyes; then
       dnl only do pkg-config magic when not cross-compiling
-      PKGTEST="yes"
+      PKGTEST="no"
     fi
-    PREFIX_OPENSSL=
     ;;
   *)
     dnl check the given --with-ssl spot
--- curl-7.67.0/libcurl.pc.in.orig	2019-12-25 23:03:57.116216398 +0300
+++ curl-7.67.0/libcurl.pc.in	2019-12-25 23:04:40.710951374 +0300
@@ -34,6 +34,7 @@
 URL: https://curl.haxx.se/
 Description: Library to transfer files with ftp, http, etc.
 Version: @CURLVERSION@
-Libs: -L${libdir} -lcurl @LIBCURL_NO_SHARED@
-Libs.private: @LIBCURL_LIBS@
-Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@
+Libs: -L${libdir} -lcurl @LIBCURL_NO_SHARED@ @LIBRESSL_LIBS@
+Libs.private: @LIBCURL_LIBS@ @LIBRESSL_LIBS_PRIVATE@
+Cflags: -I${includedir} @CPPFLAG_CURL_STATICLIB@ @LIBRESSL_CFLAGS@
+Requires.private: libressl