Improve the pkgconfig generation logic

The destination directory may not exist, so we need to mkdir just
in case.
diff --git a/Makefile b/Makefile
index 06f8ffa..37a7ba9 100644
--- a/Makefile
+++ b/Makefile
@@ -466,6 +466,7 @@
 
 
 define generate-pkgcfg
+	mkdir -p $(BLDIR)
 	echo 'Name: capstone' > $(PKGCFGF)
 	echo 'Description: Capstone disassembly engine' >> $(PKGCFGF)
 	echo 'Version: $(PKG_VERSION)' >> $(PKGCFGF)