Before using CodeBlocks the source must be configured by doing

sh ./configure

on a cosole in the same way as to when using the normal make.
That will generate the needed file linux.h.

Then CodeBlocks can be started.

CodeBlocks is mainly for development. Its useful for coding but
also for testing the source. By using Compile/all it will 
compile the source with and without optimizing and stricter
warnings.

Note: Release is set to include Debug infos - thats on purpose.
On a live server you want readable core files when it bugs.
On the other hand you want fast code, thats why its set to -O3 and
other speed settings. We trade here a bit debug overhead for the
speed but thats needed.

Its also important to the which warnings -O3 gives. The code
should be altered to work with that setting. Through the very low
depencies the server can run in heavy native opt. level. -O3 will
work fine with the current source and is tested over the years.
