Allow passing "-vv" to cargo fuzz

It should help to figure out how libFuzzer is built and linked
against, which in turn should be useful in scenarios like
google/oss-fuzz#5867 (comment)

src/options.rs

@@ -72,8 +72,8 @@ pub struct BuildOptions {
     pub debug_assertions: bool,
 
     /// Build target with verbose output from `cargo build`
-    #[structopt(short = "v", long = "verbose")]
-    pub verbose: bool,
+    #[structopt(short = "v", long = "verbose", parse(from_occurrences))]
+    pub verbose: i32,
 
     #[structopt(long = "no-default-features")]
     /// Build artifacts with default Cargo features disabled
@@ -144,8 +144,8 @@ impl stdfmt::Display for BuildOptions {
             write!(f, " -a")?;
         }
 
-        if self.verbose {
-            write!(f, " -v")?;
+        for _ in 0..self.verbose {
+            write!(f, " --verbose")?;
         }
 
         if self.no_default_features {

src/project.rs

@@ -140,8 +140,8 @@ impl FuzzProject {
         if !build.dev {
             cmd.arg("--release");
         }
-        if build.verbose {
-            cmd.arg("--verbose");
+        for _ in 0..build.verbose {
+            cmd.arg("-v");
         }
         if build.no_default_features {
             cmd.arg("--no-default-features");