sys-apps/gawk: Sync with Gentoo

It's from Gentoo commit 9fd73a4fccf890d6732d22695276991d2777888e.

sdk_container/src/third_party/portage-stable/sys-apps/gawk/gawk-5.3.1.ebuild

@@ -24,7 +24,7 @@ if [[ ${GAWK_IS_BETA} == yes || ${PV} == *_beta* ]] ; then
 	SRC_URI="https://www.skeeve.com/gawk/${MY_P}.tar.gz"
 else
 	VERIFY_SIG_OPENPGP_KEY_PATH=/usr/share/openpgp-keys/gawk.asc
-	inherit verify-sig
+	inherit verify-sig flag-o-matic
 
 	SRC_URI="mirror://gnu/gawk/${P}.tar.xz"
 	SRC_URI+=" verify-sig? ( mirror://gnu/gawk/${P}.tar.xz.sig )"
@@ -60,6 +60,8 @@ fi
 src_prepare() {
 	default
 
+	use elibc_musl && append-cppflags -D__GNU_LIBRARY__
+
 	# Use symlinks rather than hardlinks, and disable version links
 	sed -i \
 		-e '/^LN =/s:=.*:= $(LN_S):' \