Run in Linux

- Define Pi to avoid running into too many feature test macros
- Define _DEFAULT_SOURCE if not in OpenBSD
- Add a play target that works in Linux

synth.c

@@ -15,6 +15,13 @@
 
 #include <math.h>
 
+/*
+ * Apparently, it's too hard for OSes to expose M_PI in a portable way.
+ * Definition taken from
+ *	$OpenBSD: math.h,v 1.36 2018/03/10 20:52:58 kettenis Exp $
+ */
+#define JUST_PI ((double)3.14159265358979323846)
+
 #define CLAMP(x, a, b) ((x) < (a) ? (a) : (x) > (b) ? (b) : (x))
 
 double
@@ -27,7 +34,7 @@ synth_distortion(double gain, double x)
 double
 synth_sinewave(double t, double a)
 {
-	return sin(2.0 * M_PI * t * a);
+	return sin(2.0 * JUST_PI * t * a);
 }
 
 double