Add find system's swig

.travis.yml

@@ -29,6 +29,7 @@ addons:
       - python-pip
       - python2.7-dev
       - curl
+      - swig
       - graphviz
       - clang-format-3.8
       - automake

cmake/external/swig.cmake

@@ -12,16 +12,19 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-# build swig as an external project
-INCLUDE(ExternalProject)
+FIND_PACKAGE(SWIG)
 
-SET(SWIG_SOURCES_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/swig)
-SET(SWIG_INSTALL_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/install/swig)
-SET(SWIG_TARGET_VERSION "3.0.2")
-SET(SWIG_DOWNLOAD_SRC_MD5 "62f9b0d010cef36a13a010dc530d0d41")
-SET(SWIG_DOWNLOAD_WIN_MD5 "3f18de4fc09ab9abb0d3be37c11fbc8f")
+IF(NOT SWIG_FOUND)
+    # build swig as an external project
+    INCLUDE(ExternalProject)
 
-IF(WIN32)
+    SET(SWIG_SOURCES_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/swig)
+    SET(SWIG_INSTALL_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third_party/install/swig)
+    SET(SWIG_TARGET_VERSION "3.0.2")
+    SET(SWIG_DOWNLOAD_SRC_MD5 "62f9b0d010cef36a13a010dc530d0d41")
+    SET(SWIG_DOWNLOAD_WIN_MD5 "3f18de4fc09ab9abb0d3be37c11fbc8f")
+
+    IF(WIN32)
         # swig.exe available as pre-built binary on Windows:
         ExternalProject_Add(swig
             URL                 http://prdownloads.sourceforge.net/swig/swigwin-${SWIG_TARGET_VERSION}.zip
@@ -34,7 +37,7 @@ IF(WIN32)
         )
         SET(SWIG_DIR ${SWIG_SOURCES_DIR} CACHE FILEPATH "SWIG Directory" FORCE)
         SET(SWIG_EXECUTABLE ${SWIG_SOURCES_DIR}/swig.exe  CACHE FILEPATH "SWIG Executable" FORCE)
-ELSE(WIN32)
+    ELSE(WIN32)
         # From PCRE configure
         ExternalProject_Add(pcre
             ${EXTERNAL_PROJECT_LOG_ARGS}
@@ -65,6 +68,7 @@ ELSE(WIN32)
 
         SET(SWIG_DIR ${SWIG_INSTALL_DIR}/share/swig/${SWIG_TARGET_VERSION})
         SET(SWIG_EXECUTABLE ${SWIG_INSTALL_DIR}/bin/swig)
-ENDIF(WIN32)
+    ENDIF(WIN32)
 
-LIST(APPEND external_project_dependencies swig)
+    LIST(APPEND external_project_dependencies swig)
+ENDIF(NOT SWIG_FOUND)