leaked-globals: check for nm before files under missing [ci skip]

Abort if `nm` is not available, since it is needed by configure.ac to
check for prefix of external symbols.

tool/leaked-globals

@@ -18,6 +18,11 @@ end
 config = ARGV.shift
 count = 0
 col = Colorize.new
+
+# nm errors with Rust's LLVM bitcode when Rust uses a newer LLVM version than nm.
+# In case we're working with llvm-nm, tell it to not worry about the bitcode.
+no_llvm = "--no-llvm-bc" if `#{NM} --version` =~ /llvm/i
+
 config_code = File.read(config)
 REPLACE = config_code.scan(/\bAC_(?:REPLACE|CHECK)_FUNCS?\((\w+)/).flatten
 # REPLACE << 'memcmp' if /\bAC_FUNC_MEMCMP\b/ =~ config_code
@@ -46,9 +51,6 @@ end
 # darwin's ld64 seems to require exception handling personality functions to be
 # extern, so we allow the Rust one.
 REPLACE.push("rust_eh_personality") if RUBY_PLATFORM.include?("darwin")
-# nm errors with Rust's LLVM bitcode when Rust uses a newer LLVM version than nm.
-# In case we're working with llvm-nm, tell it to not worry about the bitcode.
-no_llvm = "--no-llvm-bc" if `#{NM} --version` =~ /llvm/i
 
 print "Checking leaked global symbols..."
 STDOUT.flush