config: cleanup KERNEL_CC checks, fix broken status output

If $KERNEL_CC was not defined, configure status output would print an
empty string where the kernel compiler should have been. Fix this and
simplify the code generally.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Ivan Shapovalov <[email protected]>
Closes #16997

Author: intelfx

config/always-compiler-options.m4

@@ -231,20 +231,17 @@ dnl #
 dnl # Check if kernel cc supports -Winfinite-recursion option.
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_KERNEL_CC_INFINITE_RECURSION], [
-	AC_MSG_CHECKING([whether $KERNEL_CC supports -Winfinite-recursion])
-
 	saved_cc="$CC"
+	AS_IF(
+		[ test -n "$KERNEL_CC" ], [ CC="$KERNEL_CC" ],
+		[ test -n "$KERNEL_LLVM" ], [ CC="clang" ],
+		[ CC="gcc" ]
+	)
+	AC_MSG_CHECKING([whether $CC supports -Winfinite-recursion])
+
 	saved_flags="$CFLAGS"
-	CC="gcc"
 	CFLAGS="$CFLAGS -Werror -Winfinite-recursion"
 
-	AS_IF([ test -n "$KERNEL_CC" ], [
-		CC="$KERNEL_CC"
-	])
-	AS_IF([ test -n "$KERNEL_LLVM" ], [
-		CC="clang"
-	])
-
 	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], [
 		KERNEL_INFINITE_RECURSION=-Winfinite-recursion
 		AC_DEFINE([HAVE_KERNEL_INFINITE_RECURSION], 1,
@@ -329,20 +326,17 @@ dnl #
 dnl # Check if kernel cc supports -fno-ipa-sra option.
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_ALWAYS_KERNEL_CC_NO_IPA_SRA], [
-	AC_MSG_CHECKING([whether $KERNEL_CC supports -fno-ipa-sra])
-
 	saved_cc="$CC"
+	AS_IF(
+		[ test -n "$KERNEL_CC" ], [ CC="$KERNEL_CC" ],
+		[ test -n "$KERNEL_LLVM" ], [ CC="clang" ],
+		[ CC="gcc" ]
+	)
+	AC_MSG_CHECKING([whether $CC supports -fno-ipa-sra])
+
 	saved_flags="$CFLAGS"
-	CC="gcc"
 	CFLAGS="$CFLAGS -Werror -fno-ipa-sra"
 
-	AS_IF([ test -n "$KERNEL_CC" ], [
-		CC="$KERNEL_CC"
-	])
-	AS_IF([ test -n "$KERNEL_LLVM" ], [
-		CC="clang"
-	])
-
 	AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])], [
 		KERNEL_NO_IPA_SRA=-fno-ipa-sra
 		AC_MSG_RESULT([yes])