add variant shared to yaml-cpp (#3254)

var/spack/repos/builtin/packages/yaml-cpp/package.py

@@ -33,6 +33,8 @@ class YamlCpp(CMakePackage):
 
     version('0.5.3', '4e47733d98266e46a1a73ae0a72954eb')
 
+    variant('shared', default=True,
+            description='Enable build of shared libraries')
     variant('fpic',    default=False,
             description='Build with position independent code')
 
@@ -46,5 +48,9 @@ def cmake_args(self):
             options.extend([
                 '-DCMAKE_POSITION_INDEPENDENT_CODE:BOOL=true'
             ])
+        if '+shared' in spec:
+            options.append('-DBUILD_SHARED_LIBS=ON')
+        else:
+            options.append('-DBUILD_SHARED_LIBS=OFF')
 
         return options