diff --git a/groups/010.host_deps.group b/groups/010.host_deps.group
index 0563629..0ee9174 100644
--- a/groups/010.host_deps.group
+++ b/groups/010.host_deps.group
@@ -87,6 +87,7 @@ HOST_DEPS_PREFIX="${PREFIX}";
 : ${PKG_FLEX_HOST_SHA256SUM:=e87aae032bf07c26f85ac0ed3250998c37621d95f8bd748b31f15b33c45ee995};
 : ${PKG_FLEX_HOST_VERSION:=2.6.4};
 : ${PKG_FLEX_HOST_URL:=https://github.com/westes/flex/releases/download/v${PKG_FLEX_HOST_VERSION}/flex-${PKG_FLEX_HOST_VERSION}.tar.gz};
+: ${PKG_FLEX_HOST_CFLAGS_CONFIGURE_EXTRA:="-D_GNU_SOURCE"};
 : ${PKG_GDB_HOST_DEPENDS:="chainport_host expat_host ncurses_host python2_host"};
 : ${PKG_GDB_HOST_SHA256SUM:=149abac4add030a5713f15964bd7465bef7a23712bcc91377ca40949f33e93fb};
 : ${PKG_GDB_HOST_VERSION:=7.12};
diff --git a/groups/221.native_packages_dev.group b/groups/221.native_packages_dev.group
index c8a9255..dbd32ae 100644
--- a/groups/221.native_packages_dev.group
+++ b/groups/221.native_packages_dev.group
@@ -33,6 +33,7 @@ patch patchutils perl python2 python3 ruby sbsigntools smallbasic tk")";
 : ${PKG_FLEX_SHA256SUM:=e87aae032bf07c26f85ac0ed3250998c37621d95f8bd748b31f15b33c45ee995};
 : ${PKG_FLEX_VERSION:=2.6.4};
 : ${PKG_FLEX_URL:=https://github.com/westes/flex/releases/download/v${PKG_FLEX_VERSION}/flex-${PKG_FLEX_VERSION}.tar.gz};
+: ${PKG_FLEX_CFLAGS_CONFIGURE_EXTRA:="-D_GNU_SOURCE"};
 : ${PKG_FLEX_CONFIGURE_ARGS_EXTRA:=--disable-libfl};
 : ${PKG_GDB_DEPENDS:="ncurses python2"};
 : ${PKG_GDB_SHA256SUM:=149abac4add030a5713f15964bd7465bef7a23712bcc91377ca40949f33e93fb};
diff --git a/groups/dev_packages.group b/groups/dev_packages.group
index 8a9ab76..4230856 100644
--- a/groups/dev_packages.group
+++ b/groups/dev_packages.group
@@ -62,6 +62,7 @@ DEV_PACKAGES_PYTHON="${PREFIX}/bin/python2";
 : ${PKG_FLEX_SHA256SUM:=e87aae032bf07c26f85ac0ed3250998c37621d95f8bd748b31f15b33c45ee995};
 : ${PKG_FLEX_VERSION:=2.6.4};
 : ${PKG_FLEX_URL:=https://github.com/westes/flex/releases/download/v${PKG_FLEX_VERSION}/flex-${PKG_FLEX_VERSION}.tar.gz};
+: ${PKG_FLEX_CFLAGS_CONFIGURE_EXTRA:="-D_GNU_SOURCE"};
 : ${PKG_FLEX_CONFIGURE_ARGS_EXTRA:=--disable-libfl};
 : ${PKG_BISON_SHA256SUM:=55e4a023b1b4ad19095a5f8279f0dc048fa29f970759cea83224a6d5e7a3a641};
 : ${PKG_BISON_VERSION:=3.5};