Simplify rsconf has_symbol() libc lookups

Cargo.lock

@@ -398,9 +398,9 @@ checksum = "41cc0f7e4d5d4544e8861606a285bb08d3e70712ccc7d2b84d7c0ccfaf4b05ce"
 
 [[package]]
 name = "rsconf"
-version = "0.1.1"
+version = "0.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "38a9a3f6ff97f1bc63b490ea2d2c1570fd8372ab887b45dff59695ded372d576"
+checksum = "7b9653c00e095416c7586382524a5ede22b9dec6f067879bd7eed21429721e6d"
 dependencies = [
  "cc",
 ]

Cargo.toml

@@ -42,7 +42,7 @@ serial_test = "0.4.0"
 
 [build-dependencies]
 cc = "1.0.83"
-rsconf = "0.1.1"
+rsconf = "0.1.2"
 
 [lib]
 crate-type = ["rlib"]

build.rs

@@ -88,13 +88,13 @@ fn detect_cfgs(target: Target) {
         ("gettext", &have_gettext),
         // See if libc supports the thread-safe localeconv_l(3) alternative to localeconv(3).
         ("localeconv_l", &|target| {
-            Ok(target.has_symbol_in::<String>("localeconv_l", &[]))
+            Ok(target.has_symbol("localeconv_l", None))
         }),
         ("FISH_USE_POSIX_SPAWN", &|target| {
             Ok(target.has_header("spawn.h"))
         }),
         ("HAVE_PIPE2", &|target| {
-            Ok(target.has_symbol_in::<String>("pipe2", &[]))
+            Ok(target.has_symbol("pipe2", None))
         }),
         ("HAVE_EVENTFD", &|target| {
             Ok(target.has_header("sys/eventfd.h"))