package/execline: remove -D_GNU_SOURCE

execline is written against POSIX C and automatically sets
-D_POSIX_C_SOURCE=200809L and -D_XOPEN_SOURCE=700. uclibc had a bug
consistent with the error reported in the commit that added
-D_GNU_SOURCE[1], but it has since been fixed[2].

[1]: see commit 106e43e303 ("package/execline: fix uclibc build")
[2]: https://cgit.uclibc-ng.org/cgi/cgit/uclibc-ng.git/commit/?id=354129807db12c792696b74d0cd38f2c5d2fd704

Signed-off-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/execline/execline.mk

@@ -12,7 +12,6 @@ EXECLINE_INSTALL_STAGING = YES
 EXECLINE_DEPENDENCIES = skalibs
 
 EXECLINE_CONF_OPTS = \
-	CFLAGS="$(TARGET_CFLAGS) -D_GNU_SOURCE" \
 	--with-sysdeps=$(STAGING_DIR)/usr/lib/skalibs/sysdeps \
 	--with-include=$(STAGING_DIR)/include \
 	--with-dynlib=$(STAGING_DIR)/lib \