cmake: use an external antlr2 if available

Look for an installed antlr v2, and if available use it instead of its
embedded copy.

cmake/FindAntlr2.cmake

@@ -0,0 +1,50 @@
+# - try to find Antlr v2
+# Once done this will define:
+#
+#  ANTLR2_FOUND - system has antlr2
+#  ANTLR2_INCLUDE_DIRS - the include directories for antlr2
+#  ANTLR2_LIBRARIES - Link these to use antl2
+#  ANTLR2_EXECUTABLE - The 'antlr' or 'runantlr' executable
+
+# Copyright (C) 2015, Pino Toscano, <toscano.pino@tiscali.it>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+#
+# 1. Redistributions of source code must retain the copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+# 3. The name of the author may not be used to endorse or promote products
+#    derived from this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+# OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+# IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+# INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+# NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+# THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+
+find_library(ANTLR2_LIBRARY antlr)
+set(ANTLR2_LIBRARIES "${ANTLR2_LIBRARY}")
+
+find_path(ANTLR2_INCLUDE_DIR antlr/AST.hpp)
+set(ANTLR2_INCLUDE_DIRS "${ANTLR2_INCLUDE_DIR}")
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Antlr2 DEFAULT_MSG ANTLR2_LIBRARIES ANTLR2_INCLUDE_DIRS)
+
+find_program(ANTLR2_EXECUTABLE NAMES runantlr runantlr2 antlr)
+
+mark_as_advanced(
+  ANTLR2_INCLUDE_DIRS
+  ANTLR2_LIBRARIES
+  ANTLR2_EXECUTABLE
+)