glsl/apps: print usage info if insufficient args
diff --git a/src/glsl/apps/process.c b/src/glsl/apps/process.c
index 2d2ab91..e65f35c 100644
--- a/src/glsl/apps/process.c
+++ b/src/glsl/apps/process.c
@@ -47,6 +47,7 @@
    unsigned int i;
 
    if (argc != 3) {
+      printf("Usage: process infile outfile\n");
       return 1;
    }
 
diff --git a/src/glsl/apps/purify.c b/src/glsl/apps/purify.c
index 8c01f4f..3019e8b 100644
--- a/src/glsl/apps/purify.c
+++ b/src/glsl/apps/purify.c
@@ -45,6 +45,7 @@
    FILE *out;
 
    if (argc != 3) {
+      printf("Usage: purify infile outfile\n");
       return 1;
    }
 
diff --git a/src/glsl/apps/tokenise.c b/src/glsl/apps/tokenise.c
index 9ff7315..c70c3cc 100644
--- a/src/glsl/apps/tokenise.c
+++ b/src/glsl/apps/tokenise.c
@@ -46,6 +46,7 @@
    unsigned int i;
 
    if (argc != 3) {
+      printf("Usage: tokenize infile outfile\n");
       return 1;
    }
 
diff --git a/src/glsl/apps/version.c b/src/glsl/apps/version.c
index 40a4a06..0420f97 100644
--- a/src/glsl/apps/version.c
+++ b/src/glsl/apps/version.c
@@ -45,6 +45,7 @@
    FILE *out;
 
    if (argc != 3) {
+      printf("Usage: version infile outfile\n");
       return 1;
    }