Disable getloadavg support by default

Many platforms don't have getloadavg or hide it behind platform-specific
defines. Rather than try to work around it with an ifdef maze,
disable it by default. Builders can enable it by adding
`-D HAVE_LOADAVG` to their CFLAGS, along with whatever other defines
are needed for their platform.

README.md

@@ -19,9 +19,10 @@ It is feature-complete and supports most of the same options as ninja.
 
 samurai requires various POSIX.1-2008 interfaces.
 
-Scheduling jobs based on load average requires through the non-standard, but
-widely available `getloadavg` function. This feature can be disabled by
-defining the `NO_GETLOADAVG` macro when calling the C compiler.
+Scheduling jobs based on load average requires the non-standard
+`getloadavg` function. This feature can be enabled by defining
+`HAVE_GETLOADAVG` in your `CFLAGS`, along with any other necessary
+definitions for your platform.
 
 ## Differences from ninja
 

build.c

@@ -1,7 +1,4 @@
 #define _POSIX_C_SOURCE 200809L
-#ifndef NO_GETLOADAVG
-#define _BSD_SOURCE /* for getloadavg */
-#endif
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
@@ -527,9 +524,7 @@ jobwork(struct job *j)
 static double
 queryload(void)
 {
-#ifdef NO_GETLOADAVG
-	return 0;
-#else
+#ifdef HAVE_GETLOADAVG
 	double load;
 
 	if (getloadavg(&load, 1) == -1) {
@@ -538,6 +533,8 @@ queryload(void)
 	}
 
 	return load;
+#else
+	return 0;
 #endif
 }
 

samu.c

@@ -53,9 +53,7 @@ debugflag(const char *flag)
 static void
 loadflag(const char *flag)
 {
-#ifdef NO_GETLOADAVG
-	warn("job scheduling based on load average is not implemented");
-#else
+#ifdef HAVE_GETLOADAVG
 	double value;
 	char *end;
 	errno = 0;
@@ -64,6 +62,8 @@ loadflag(const char *flag)
 	if (*end || value < 0 || errno != 0)
 		fatal("invalid -l parameter");
 	buildopts.maxload = value;
+#else
+	warn("job scheduling based on load average is not supported");
 #endif
 }