argp-standalone: fix build with gcc 7.x

argp-standalone: fix build with gcc 7.x

Back in commit a662ff7e79630ca0875dd8529fe54db27a275007
("package/argp-standalone: Fix build with c99 compilers"), we fixed
the build of argp-standalone with compilers defaulting to C99 inline
semantics, i.e starting from gcc 5.x.

This was done as part of a patch that used "inline" instead of "extern
inline". However, using "inline" once again broke the build with gcc
7.x. To fix this, revert back to using just "extern inline" (hence
removing a patch of patch 0003-fix_build_with_c99_compilers.patch) and
instead use -fgnu89-inline in the CFLAGS.

See https://gcc.gnu.org/gcc-5/porting_to.html for more details.

Fixes:

  http://autobuild.buildroot.net/results/a9cedc54829b7bd2dd7ae6ff2bd6c6db242f1c35/

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Thomas: also drop the patch of
0003-fix_build_with_c99_compilers.patch that is no longer needed.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>