Kdevelop4 likes cmake as the make system.
When I brought a simple C program into Kdevelop4, I did as a4z suggested and created a new C++ project and exited Kdevelop4.
Then I edited the toplevel CMakeLists.txt file. Please note the C after the <projectname> in the PROJECT line.
Code:
PROJECT(<projectname> C)
ADD_SUBDIRECTORY(src)
I then copied in the include and src directories from my previous project.
In the src directory I added a CMakeLists.txt file
Code:
PROJECT(<projectname> C)
ADD_EXECUTABLE(<projectname> <space delimited list of .c files>)
INCLUDE_DIRECTORIES (../include)
Now the project can be opened and used successfully in Kdevelop4.