Find a working C compiler
Sets the variable CC if it is not already set
Unlike AC_PROG_CC, this does not prefer gcc and does not set CFLAGS.
It does check that the compiler can compile a simple C program.
It also sets the variable GCC to yes if the compiler is gcc. It does
not yet check for some special options needed in particular for
parallel computers, such as -Tcray-t3e, or special options to get
full ANSI/ISO C, such as -Aa for HP.