build: fix musl-targeting toolchain test

the old test was broken in that it would never fail on a toolchains built
without dynamic linking support, leading to the wrapper script possibly being
installed on compilers that do not support it. in addition, the new test is
portable across compilers: the old test only worked on GCC.

the new test works by testing whether the toolchain libc defines __GLIBC__:
most non-musl Linux libc's do define this for compatibility even when they
are not glibc, so this is a safe bet to check for musl. in addition, the
compiler runtime would need to have a somewhat glibc-compatible ABI in the
first place, so any non-glibc compatible libc's compiler runtime might not
work. it is safer to disable these cases by default and have the user enable
the wrappers manually there using --enable-wrapper if they certain it works.

configure

@@ -237,17 +237,16 @@ echo "$cc_family"
 # Figure out toolchain wrapper to build
 #
 if test "$wrapper" = auto -o "$wrapper" = detect ; then
+echo "#include <stdlib.h>" > "$tmpc"
+echo "#if ! __GLIBC__" >> "$tmpc"
+echo "#error no" >> "$tmpc"
+echo "#endif" >> "$tmpc"
 printf "checking for toolchain wrapper to build... "
-if test "$cc_family" = gcc ; then
+if test "$wrapper" = auto && ! $CC -c -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
+echo "none"
+elif test "$cc_family" = gcc ; then
 gcc_wrapper=yes
-if test "$wrapper" = auto ; then
-while read line ; do
-case "$line" in */ld-musl-*) gcc_wrapper=no ;; esac
-done <<EOF
-$($CC -dumpspecs)
-EOF
-fi
-test "$gcc_wrapper" = yes && echo "gcc"
+echo "gcc"
 else
 echo "none"
 if test "$wrapper" = detect ; then