dev-libs/libmodbus: respect users cflags

Closes: https://bugs.gentoo.org/941995
Signed-off-by: Conrad Kostecki <conikost@gentoo.org>

dev-libs/libmodbus/files/libmodbus-3.1.11-cflags.patch

@@ -0,0 +1,17 @@
+--- a/configure.ac
++++ b/configure.ac
+@@ -178,14 +178,6 @@
+   [enable_debug=$enableval],
+   [enable_debug=no])
+ 
+-AS_IF([test "x$enable_debug" = "xyes"], [
+-  CFLAGS="-g -O0"
+-  CXXFLAGS="-g -O0"
+-], [
+-  CFLAGS="-O2"
+-  CXXFLAGS="-O2"
+-])
+-
+ AC_OUTPUT
+ AC_MSG_RESULT([
+         $PACKAGE $VERSION

dev-libs/libmodbus/libmodbus-3.1.11-r1.ebuild

@@ -3,6 +3,8 @@
 
 EAPI=8
 
+inherit autotools
+
 DESCRIPTION="Modbus library which supports RTU communication over a serial line or a TCP link"
 HOMEPAGE="https://libmodbus.org/"
 SRC_URI="https://github.com/stephane/${PN}/releases/download/v${PV}/${P}.tar.gz"
@@ -13,6 +15,13 @@ KEYWORDS="~amd64 ~arm ~arm64 ~ppc ~ppc64 ~riscv ~x86"
 IUSE="static-libs test"
 RESTRICT="!test? ( test )"
 
+PATCHES=( "${FILESDIR}/${PN}-3.1.11-cflags.patch" )
+
+src_prepare() {
+	default
+	eautoreconf
+}
+
 src_configure() {
 	local myeconfargs=(
 		$(use_enable test tests)