Simplify (and fix?) build.rs HAVE_XXX detection

Since none of the compiles(xxx) calls are to particularly complex code, we can
just use `rsconf` directly to test for the presence of the symbols or headers as
needed.

Note that it seems at least some of the previous detection was not working
correctly; in particular HAVE_PIPE2 was evaluating to false on my WSL install
where pipe2(2) was available (caught because it revealed some compilation errors
in that conditional compilation path after porting).

I kept the cfg names and the tests themselves mostly as-is, though we might want
to change that to conform with the rust convention of lowercase cfg names and
decide whether we want to prefix all these with have_, fish_, or nothing at all.
Also the posix_spawn() test should probably check for the symbol `posix_spawn()`
rather than the header `spawn.h` since we don't use it via the header but rather
via the symbol (but in reality they're almost certainly going to give the same
result).

build.rs

@@ -55,19 +55,6 @@ fn main() {
         .include(&build_dir)
         .compile("flibc.a");
 
-    if compiles("src/cfg/w_exitcode.cpp") {
-        println!("cargo:rustc-cfg=HAVE_WAITSTATUS_SIGNAL_RET");
-    }
-    if compiles("src/cfg/eventfd.c") {
-        println!("cargo:rustc-cfg=HAVE_EVENTFD");
-    }
-    if compiles("src/cfg/pipe2.c") {
-        println!("cargo:rustc-cfg=HAVE_PIPE2");
-    }
-    if compiles("src/cfg/spawn.c") {
-        println!("cargo:rustc-cfg=FISH_USE_POSIX_SPAWN");
-    }
-
     let mut build = cc::Build::new();
     // Add to the default library search path
     build.flag_if_supported("-L/usr/local/lib/");
@@ -102,6 +89,10 @@ fn detect_cfgs(target: Target) {
         ("gettext", &have_gettext),
         // See if the system headers provide the thread-safe localeconv_l(3) alternative to localeconv(3).
         ("localeconv_l", &|target| Ok(target.has_symbol_in::<String>("localeconv_l", &[]))),
+        ("FISH_USE_POSIX_SPAWN", &|target| Ok(target.has_header("spawn.h"))),
+        ("HAVE_PIPE2", &|target| Ok(target.has_symbol_in::<String>("pipe2", &[]))),
+        ("HAVE_EVENTFD", &|target| Ok(target.has_header("sys/eventfd.h"))),
+        ("HAVE_WAITSTATUS_SIGNAL_RET", &|target| Ok(target.r#if("WEXITSTATUS(0x007f) == 0x7f", "sys/wait.h"))),
     ] {
         match handler(&target) {
             Err(e) => rsconf::warn!("{}: {}", name, e),

src/cfg/eventfd.c

@@ -1 +0,0 @@
-#include <sys/eventfd.h>

src/cfg/pipe2.c

@@ -1,2 +0,0 @@
-#include <unistd.h>
-int ok = pipe2(0, 0);

src/cfg/spawn.c

@@ -1 +0,0 @@
-#include <spawn.h>

src/cfg/w_exitcode.cpp

@@ -1,2 +0,0 @@
-#include <sys/wait.h>
-static_assert(WEXITSTATUS(0x007f) == 0x7f, "");