bpo-33483: more correctly handle finding the C compiler (GH-6780)

Instead of passing configure args such as --without-gcc or --with-icc,
instead prefer to rely on the native way of finding the compiler:
passing CC (or CPP or CXX depending).

This allows configure to find the correct compiler instead of having to
be explicitly told. It also more correctly builds on both macOS and
FreeBSD since the system compiler is used by default (cc)

Misc/NEWS.d/next/Build/2018-05-13-17-21-54.bpo-33483.WOs-en.rst

@@ -0,0 +1,2 @@
+C compiler is now correctly detected from the standard environment
+variables. --without-gcc and --with-icc options have been removed.

configure.ac

@@ -555,43 +555,6 @@ EXPORT_MACOSX_DEPLOYMENT_TARGET='#'
 # when running configure or make.  The build should not break if they do.
 # BASECFLAGS should generally not be messed with, however.
 
-# XXX shouldn't some/most/all of this code be merged with the stuff later
-# on that fiddles with OPT and BASECFLAGS?
-AC_MSG_CHECKING(for --without-gcc)
-AC_ARG_WITH(gcc,
-            AS_HELP_STRING([--without-gcc], [never use gcc]),
-[
-	case $withval in
-	no)	CC=${CC:-cc}
-		without_gcc=yes;;
-	yes)	CC=gcc
-		without_gcc=no;;
-	*)	CC=$withval
-		without_gcc=$withval;;
-	esac], [
-	case $ac_sys_system in
-	AIX*)   CC=${CC:-xlc_r}
-		without_gcc=;;
-	*)	without_gcc=no;;
-	esac])
-AC_MSG_RESULT($without_gcc)
-
-AC_MSG_CHECKING(for --with-icc)
-AC_ARG_WITH(icc,
-            AS_HELP_STRING([--with-icc], [build with icc]),
-[
-	case $withval in
-	no)	CC=${CC:-cc}
-		with_icc=no;;
-	yes)	CC=icc
-		CXX=icpc
-		with_icc=yes;;
-	*)	CC=$withval
-		with_icc=$withval;;
-	esac], [
-	with_icc=no])
-AC_MSG_RESULT($with_icc)
-
 # If the user switches compilers, we can't believe the cache
 if test ! -z "$ac_cv_prog_CC" -a ! -z "$CC" -a "$CC" != "$ac_cv_prog_CC"
 then
@@ -1749,7 +1712,6 @@ yes)
 	    BASECFLAGS="$BASECFLAGS -m486 -DSCO5"
 	    ;;
 
-    # is there any other compiler on Darwin besides gcc?
     Darwin*)
         # -Wno-long-double, -no-cpp-precomp, and -mno-fused-madd
         # used to be here, but non-Apple gcc doesn't accept them.