ENH: Prevent cmake in source builds

Building directly inside the root of the source tree
can cause problems where the build intermediate files
overwrite or conflict with the intended source code
files.

This modification identifies this problem and
issues failure messages and suggestions to over
come the problem with more robust build suggestion.

Author: hjmjohnson

Diff for: CMakeLists.txt

@@ -76,6 +76,9 @@ project(JSONCPP
 message(STATUS "JsonCpp Version: ${JSONCPP_VERSION_MAJOR}.${JSONCPP_VERSION_MINOR}.${JSONCPP_VERSION_PATCH}")
 set( JSONCPP_SOVERSION 22 )
 
+include(${CMAKE_CURRENT_SOURCE_DIR}/include/PreventInSourceBuilds.cmake)
+include(${CMAKE_CURRENT_SOURCE_DIR}/include/PreventInBuildInstalls.cmake)
+
 option(JSONCPP_WITH_TESTS "Compile and (for jsoncpp_check) run JsonCpp test executables" ON)
 option(JSONCPP_WITH_POST_BUILD_UNITTEST "Automatically run unit-tests as a post build step" ON)
 option(JSONCPP_WITH_WARNING_AS_ERROR "Force compilation to fail if a warning occurs" OFF)

Diff for: include/PreventInBuildInstalls.cmake

@@ -0,0 +1,9 @@
+string(TOLOWER "${CMAKE_INSTALL_PREFIX}" _PREFIX)
+string(TOLOWER "${ITK_BINARY_DIR}" _BUILD)
+if("${_PREFIX}" STREQUAL "${_BUILD}")
+  message(FATAL_ERROR
+    "The current CMAKE_INSTALL_PREFIX points at the build tree:\n"
+    "  ${CMAKE_INSTALL_PREFIX}\n"
+    "This is not supported."
+    )
+endif()

Diff for: include/PreventInSourceBuilds.cmake

@@ -0,0 +1,45 @@
+#
+# This function will prevent in-source builds
+function(AssureOutOfSourceBuilds)
+  # make sure the user doesn't play dirty with symlinks
+  get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+  get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+
+  # disallow in-source builds
+  if("${srcdir}" STREQUAL "${bindir}")
+    message("######################################################")
+    message("# jsoncpp should not be configured & built in the jsoncpp source directory")
+    message("# You must run cmake in a build directory.")
+    message("# For example:")
+    message("# mkdir jsoncpp-Sandbox ; cd jsoncpp-sandbox")
+    message("# git clone https://github.com/open-source-parsers/jsoncpp.git # or download & unpack the source tarball")
+    message("# mkdir jsoncpp-build")
+    message("# this will create the following directory structure")
+    message("#")
+    message("# jsoncpp-Sandbox")
+    message("#  +--jsoncpp")
+    message("#  +--jsoncpp-build")
+    message("#")
+    message("# Then you can proceed to configure and build")
+    message("# by using the following commands")
+    message("#")
+    message("# cd jsoncpp-build")
+    message("# cmake ../jsoncpp # or ccmake, or cmake-gui ")
+    message("# make")
+    message("#")
+    message("# NOTE: Given that you already tried to make an in-source build")
+    message("#       CMake have already created several files & directories")
+    message("#       in your source tree. run 'git status' to find them and")
+    message("#       remove them by doing:")
+    message("#")
+    message("#       cd jsoncpp-Sandbox/jsoncpp")
+    message("#       git clean -n -d")
+    message("#       git clean -f -d")
+    message("#       git checkout --")
+    message("#")
+    message("######################################################")
+    message(FATAL_ERROR "Quitting configuration")
+  endif()
+endfunction()
+
+AssureOutOfSourceBuilds()