Don't unreliably attempt to find command executables.

Also fix a typo in a diagnostic.

Index: build-tools/icon-theme-installer
--- build-tools/icon-theme-installer.orig
+++ build-tools/icon-theme-installer
@@ -99,17 +99,17 @@ if test -z "$THEME_NAME"; then
 fi
 
 if test -z "$INSTALL_BASE_DIR"; then
-	echo "Base theme directory required [-d \$(hicolordir)]"
+	echo "Base theme directory required [-b \$(hicolordir)]"
 	exit 1
 fi
 
-if test ! -x `echo "$MKINSTALLDIRS_EXEC" | cut -f1 -d' '`; then
-	echo "Cannot find '$MKINSTALLDIRS_EXEC'; You probably want to pass -m \$(mkinstalldirs)"
+if test -z "$MKINSTALLDIRS_EXEC"; then
+	echo "mkinstalldirs command required [-m \$(mkinstalldirs)]"
 	exit 1
 fi
 
-if test ! -x $(which `echo "$INSTALL_DATA_EXEC" | cut -f1 -d' '`); then
-	echo "Cannot find '$INSTALL_DATA_EXEC'; You probably want to pass -x \$(INSTALL_DATA)"
+if test -z "$INSTALL_DATA_EXEC"; then
+	echo "INSTALL_DATA command required [-x \$(INSTALL_DATA)]"
 	exit 1
 fi