[librsvg/rustification] Visual Studio builds: Use a default value for $(CARGO)

[Chun-wei Fan <fanchunwei src gnome org>]

commit 5a557714ae0a18dfad097c8f91f1703ce62b7934
Author: Chun-wei Fan <fanchunwei src gnome org>
Date:   Fri Dec 16 14:32:09 2016 +0800

    Visual Studio builds: Use a default value for $(CARGO)
    
    Define $(CARGO) to be simply 'cargo' unless it is overridden when we use the
    NMake Makefile to build the rust bits using Visual Studio.

 build/win32/rsvg-rust.mak |    6 +-----
 1 files changed, 1 insertions(+), 5 deletions(-)
---
diff --git a/build/win32/rsvg-rust.mak b/build/win32/rsvg-rust.mak
index 36af547..e33e62a 100644
--- a/build/win32/rsvg-rust.mak
+++ b/build/win32/rsvg-rust.mak
@@ -1,9 +1,7 @@
 !include detectenv-msvc.mak
 
 !if "$(CARGO)" == ""
-BUILD_RUST = 0
-!else
-BUILD_RUST = 1
+CARGO = cargo
 !endif
 
 !if "$(VALID_CFGSET)" == "TRUE"
@@ -40,7 +38,5 @@ clean:
 !else
 !if "$(VALID_CFGSET)" == "FALSE"
 !error You need to specify an appropriate config for your build, using CFG=Release|Debug
-!else
-!error You need to specify an appropriate path for your cargo executable using CARGO=<path_to_cargo.exe>
 !endif
 !endif