add openssl-nolink feature to avoid linking the openssl library

This is needed to link against boringssl, but against the library
built statically by the boring-sys crate. This requires the caller to
depend on boring-sys to work, but this isn't really an issue since it
wouldn't even need this feature if that is not the case.

Cargo.toml

@@ -29,6 +29,7 @@ module-hash = ["yara-sys/module-hash"]
 profiling = ["yara-sys/profiling"]
 ndebug = ["yara-sys/ndebug"]
 openssl-static = ["yara-sys/openssl-static"]
+openssl-nolink = ["yara-sys/openssl-nolink"]
 yara-static = ["yara-sys/yara-static"]
 
 [dependencies]

yara-sys/Cargo.toml

@@ -25,6 +25,7 @@ module-hash = []
 profiling = []
 ndebug = []
 openssl-static = []
+openssl-nolink = []
 yara-static = []
 
 [build-dependencies]

yara-sys/build.rs

@@ -113,6 +113,7 @@ mod build {
         } else if cfg!(feature = "openssl-static") {
             println!("cargo:rustc-link-lib=static=ssl");
             println!("cargo:rustc-link-lib=static=crypto");
+        } else if cfg!(feature = "openssl-nolink") {
         } else {
             println!("cargo:rustc-link-lib=dylib=ssl");
             println!("cargo:rustc-link-lib=dylib=crypto");