Use cYAML if it is available in Python. (#2010)

bin/spack

@@ -40,6 +40,16 @@ SPACK_PREFIX = os.path.dirname(os.path.dirname(SPACK_FILE))
 # Allow spack libs to be imported in our scripts
 SPACK_LIB_PATH = os.path.join(SPACK_PREFIX, "lib", "spack")
 sys.path.insert(0, SPACK_LIB_PATH)
+
+# Try to use system YAML if it is available, as it might have libyaml
+# support (for faster loading via C).  Load it before anything in
+# lib/spack/external so it will take precedence over Spack's PyYAML.
+try:
+    import yaml
+except ImportError:
+    pass   # ignore and use slow yaml
+
+# Add external libs
 SPACK_EXTERNAL_LIBS = os.path.join(SPACK_LIB_PATH, "external")
 sys.path.insert(0, SPACK_EXTERNAL_LIBS)