CI: Let CMake use the CC environment variable

CC from environment is used to initialize CMAKE_C_COMPILER so
setting CMAKE_C_COMPILER explicitly isn't needed.

The syntax in ci_build.bash was broken in case one wished to put
spaces in CC.

build-aux/ci_build.bash

@@ -242,10 +242,6 @@ then
 
 		add_extra_option "$SMALL" "-DXZ_SMALL=ON" ""
 
-		if test -n "$CC" ; then
-			EXTRA_OPTIONS="$EXTRA_OPTIONS -DCMAKE_C_COMPILER=$CC"
-		fi
-
 		# Remove old cache file to clear previous settings.
 		rm -f "CMakeCache.txt"
 		cmake "$SRC_DIR/CMakeLists.txt" -B "$DEST_DIR" $EXTRA_OPTIONS -DXZ_CHECKS="$CHECK_TYPE" -G "Unix Makefiles"