In order to be able to build V8 from scratch on Linux/Mac for x64 please heed the following steps:
More in-depth information can be found here.
1.) Install Git
3.) Update depot_tools by executing the following into your terminal/shell:
gclient
4.) Go into the directory where you want to download the V8 source into and execute the following in your terminal/shell:
fetch v8 cd v8
Don't simply git-clone the V8 repository!
More in-depth information can be found in the here.
5.) Make sure that you are in the V8 source directory. If you followed every step up until step 4.) you already should be at the right location.
6.) Download all the build dependencies by executing the following in your terminal/shell:
gclient sync
7.) Generate the necessary build files by executing the following in your terminal/shell:
tools/dev/v8gen.py x64.release
8.) Compile the source by executing the following in your terminal/shell:
ninja -C out.gn/x64.release
9.) Run the tests by executing the following in your terminal/shell:
tools/run-tests.py --gn