diff --git a/gen.lua b/gen.lua
index 77431aaa..bc11b49e 100644
--- a/gen.lua
+++ b/gen.lua
@@ -20,6 +20,7 @@ include 'rules.ninja'
 
 toolchain 'target'
 
+subgen 'probe'
 subgen 'pkg'
 subgen 'src'
 
diff --git a/probe/gen.lua b/probe/gen.lua
new file mode 100644
index 00000000..641443d1
--- /dev/null
+++ b/probe/gen.lua
@@ -0,0 +1,3 @@
+local function probe(var)
+	build('probe', '$outdir/'..var, '$dir/'..var, {var=var})
+end
diff --git a/rules.ninja b/rules.ninja
index 19458c89..8d02c75d 100644
--- a/rules.ninja
+++ b/rules.ninja
@@ -39,6 +39,9 @@ rule mc
 rule muse
   command = muse $museflags -o $out $in
 
+rule probe
+  command = rc ./scripts/probe.rc $var $cc $cflags $ldflags -o /dev/null -x c $in >$out.tmp && mv $out.tmp $out
+
 # misc tools
 rule touch
   command = touch $out
diff --git a/scripts/probe.rc b/scripts/probe.rc
new file mode 100644
index 00000000..aba1099d
--- /dev/null
+++ b/scripts/probe.rc
@@ -0,0 +1,7 @@
+flag e +
+
+var=$1
+shift
+if($* >[2]/dev/null)
+	echo '#define '$var' 1'
+status=()