[PATCH] Silence clang -Wignored-attributes when building with clang

Building glibc with clang trips -Wignored-attributes on the alias
redirections glibc uses intentionally, e.g. the long_double_symbol
machinery where a public name aliases a weak alias of an internal
symbol.

These warnings are harmless: the alias attribute still generates
the correct ELF symbol table entries, and the GNU linker resolves
indirect alias chains at link time.  Clang warns that attributes
on the intermediate alias target won't be inherited by the outer
alias, but that attribute propagation is neither expected nor
needed here -- the aliasing semantics themselves are unaffected.

Detect whether CC is clang (cc-clang) and add -Wno-ignored-attributes
to the warning flags.
---
 Makeconfig   |  4 ++++
 configure    | 29 +++++++++++++++++++++++++++++
 configure.ac |  9 +++++++++
 3 files changed, 42 insertions(+)

diff --git ./Makeconfig ./Makeconfig
index 8fe7217dd7..4c34e59aed 100644
--- ./Makeconfig
+++ ./Makeconfig
@@ -908,6 +908,10 @@ endif
 ifeq ($(enable-werror),yes)
 +gccwarn += -Werror
 endif
+# Clang warns on the intentional alias redirections (e.g. long_double_symbol).
+ifeq ($(cc-clang),yes)
++gccwarn += -Wno-ignored-attributes
+endif
 +gccwarn-c = -Wstrict-prototypes -Wold-style-definition \
   $(cc-option-wfree-labels) $(cc-option-wmissing-parameter-name)
 
diff --git ./configure ./configure
index 90c4b4f858..fbe8e4ac30 100755
--- ./configure
+++ ./configure
@@ -6031,6 +6031,35 @@ config_vars="$config_vars
 have-test-clang = $libc_cv_test_clang"
 
 
+cat > conftest.c <<EOF
+$conftest_code
+EOF
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether CC is clang" >&5
+printf %s "checking whether CC is clang... " >&6; }
+if test ${libc_cv_cc_clang+y}
+then :
+  printf %s "(cached) " >&6
+else case e in #(
+  e)   if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -c conftest.c -o conftest 1>&5'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }
+  then
+    libc_cv_cc_clang=yes
+  else
+    libc_cv_cc_clang=no
+  fi ;;
+esac
+fi
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $libc_cv_cc_clang" >&5
+printf "%s\n" "$libc_cv_cc_clang" >&6; }
+rm -f conftest*
+config_vars="$config_vars
+cc-clang = $libc_cv_cc_clang"
+
+
 cat > conftest.cc <<EOF
 $conftest_code
 EOF
diff --git ./configure.ac ./configure.ac
index 2a788c13b7..0d81a6e7a0 100644
--- ./configure.ac
+++ ./configure.ac
@@ -651,6 +651,15 @@ LIBC_TRY_TEST_CC_COMMAND([for clang],
 )
 LIBC_CONFIG_VAR([have-test-clang], [$libc_cv_test_clang])
 
+dnl Check whether the library itself is built with clang (CC).
+LIBC_TRY_CC_COMMAND([whether CC is clang],
+  [$conftest_code],
+  [-c],
+  libc_cv_cc_clang,
+  [libc_cv_cc_clang=yes], [libc_cv_cc_clang=no],
+)
+LIBC_CONFIG_VAR([cc-clang], [$libc_cv_cc_clang])
+
 dnl Check if clang++ is used to test glibc.
 LIBC_TRY_TEST_CXX_COMMAND([for clang++],
   [$conftest_code],
-- 
2.54.0