build: Change return type of main in configure compile test programs

When running the configure script with `CC=clang ./configure' clang
fails all the compile test programs due to the main() function not
having return type int. Change it, so we can use clang as a compiler in
the configure script.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: travis: Re-enable clang and also use it for configure

Make sure we also use clang for the compile tests in the configure
script.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: Detect compiler (gcc or clang) and filter out compiler flags

Create a make variable $(COMPILER) which is set to either `gcc' or
`clang' depending on the compiler used. Use this variable to filter out
command line options not supported by clang (for now only
-fno-delete-null-pointer-checks).

Compiler detection snippet extracted from the Linux kernel Makefile.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>