configure: Fix check for INSERT in linker script

GCC/Clang use local access when referencing a const variable,
so the conftest.so may have no dynamic relocation.
LLD reports `error: unable to insert .foo after .rela.dyn` when the
destination section does not exist.

Use a non-const int to ensure that .rela.dyn exists.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

configure

@@ -5793,7 +5793,7 @@ if ${libc_cv_insert+:} false; then :
   $as_echo_n "(cached) " >&6
 else
   cat > conftest.c <<EOF
-		const int __attribute__ ((section(".bar"))) bar = 0x12345678;
+		int __attribute__ ((section(".bar"))) bar = 0x12345678;
 		int test (void) { return bar; }
 EOF
 		cat > conftest.t <<EOF

configure.ac

@@ -1217,7 +1217,7 @@ AC_SUBST(libc_cv_protected_data)
 AC_CACHE_CHECK(linker support for INSERT in linker script,
 	       libc_cv_insert,
 	       [cat > conftest.c <<EOF
-		const int __attribute__ ((section(".bar"))) bar = 0x12345678;
+		int __attribute__ ((section(".bar"))) bar = 0x12345678;
 		int test (void) { return bar; }
 EOF
 		cat > conftest.t <<EOF