Add Cotire Pch + cling interpreter script

- Generate a feel++ bash script that configure cling (system)
  use "-DCLING_PREFIX=/path/to/prefix/cling" if cling is installed in a non standard binary path.
- Add cotire to the project to generate precompiled headers (pch).
  Introduce FEELPP_ENABLE_PCH and FEELPP_ENABLE_PCH_APPLICATIONS
  options.

.gitignore

@@ -5,6 +5,8 @@ build/
 *.log
 *.orig
 *.vim
+*.pch
+*.gch
 /#CTestConfig.cmake#
 /#ctest.make#
 /.DS_Store

cmake/modules/FindCling.cmake

@@ -0,0 +1,42 @@
+# - Find Feel
+# This module looks for Feel (Library for the Finite Element Method) support
+# it will define the following values
+#  CLING_FOUND = set if FEELPP has been found
+#  CLING_INCLUDE_DIR = where feel/feel.hpp can be found
+#  CLING_LIBRARY    = the library to link in
+
+find_path(CLING_INCLUDE_DIR ClingOptions.h
+    HINTS
+    ${CLING_PREFIX}/include/cling/Interpreter
+    /usr/include/cling/Interpreter
+    /usr/local/include/cling/Interpreter
+    /opt/local/include/cling/Interpreter
+    NO_DEFAULT_PATH
+)
+
+find_library(CLING_LIBRARIES cling
+    HINTS
+    ${CLING_PREFIX}/lib
+    /usr/lib
+    /usr/local/lib
+    /opt/local/lib
+    NO_DEFAULT_PATH
+)
+
+if( CLING_PREFIX )
+    message( STATUS, "[cling] prefix dir: ${CLING_PREFIX}" )
+endif()
+
+find_program(CLING_BIN cling
+    HINTS
+    ${CLING_PREFIX}/bin
+    $PATH
+    CMAKE_SYSTEM_PROGRAM_PATH
+)
+
+message( STATUS "[cling] include dir: ${CLING_INCLUDE_DIR}" )
+message( STATUS "[cling] libraries: ${CLING_LIBRARIES}" )
+message( STATUS "[cling] binary: ${CLING_BIN}" )
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS( Cling REQUIRED_VARS CLING_BIN )