Change to out-of source build, Update build instructions (#3271)

* update build instructions

Usage of "modern" CMake syntax and using specific out of source build.
Further add example on how to build in parallel, build a specific target
and how to change the compiler.

This closes #3236.

.gitignore

@@ -1,24 +1,12 @@
 cmake.h
 commit.h
-Makefile
-src/task
-src/taskd
-src/libtask.a
-src/commands/libcommands.a
-src/tc/libtc.a
-src/columns/libcolumns.a
 *~
 .*.swp
 Session.vim
 package-config/osx/binary/task
-/build/
-CMakeFiles
-CMakeCache.txt
-cmake_install.cmake
+/build*/
 install_manifest.txt
 _CPack_Packages
-CPackConfig.cmake
-CPackSourceConfig.cmake
 patches
 *.exe
 tutorials