package/boinc: fix build with autoconf >= 2.70

Fix the following build failure raised since bump of autoconf to version
2.71 in commit ecd54b65c1f998a7ccd91f7c523e4ff38c4781da and
http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=commit;h=ec90049dfcf4538750e61d675d885157fa5ca7f8:

checking for /home/buildroot/autobuild/instance-0/output-1/host/bin/mips-linux-gnu-gcc options needed to detect all undeclared functions... cannot detect
configure: error: in `/home/buildroot/autobuild/instance-0/output-1/build/boinc-7.16.18':
configure: error: cannot make /home/buildroot/autobuild/instance-0/output-1/host/bin/mips-linux-gnu-gcc report undeclared builtins

This error raised because AC_CHECK_DECLS returns an error due to -mavx
flag:

mips-linux-gnu-gcc: error: unrecognized command line option '-mavx'

To fix this build failure, hard code ac_cv_c_undeclared_builtin_options
to 'none needed' as removing '-mavx' from CPPFLAGS will hinder the
detection of xgetbv, xgetbv, __xgetbv, cpuid, _cpuid and __cpuid

Fixes:
 - http://autobuild.buildroot.org/results/4b468a5d3bfbff9638316ca930ef791867774aef

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/boinc/boinc.mk

@@ -14,7 +14,15 @@ BOINC_CPE_ID_VENDOR = rom_walton
 BOINC_SELINUX_MODULES = boinc
 BOINC_DEPENDENCIES = host-pkgconf libcurl openssl
 BOINC_AUTORECONF = YES
-BOINC_CONF_ENV = ac_cv_path__libcurl_config=$(STAGING_DIR)/usr/bin/curl-config
+# The ac_cv_c_undeclared_builtin_options value is to help
+# AC_CHECK_DECLS realize that it doesn't need any particular compiler
+# option to get an error when building a program that uses undeclared
+# symbols. Otherwise, AC_CHECK_DECLS is confused by the configure
+# script unconditionally passing -mavx, which only exists on x86, and
+# therefore causes a failure on all other architectures.
+BOINC_CONF_ENV = \
+	ac_cv_c_undeclared_builtin_options='none needed' \
+	ac_cv_path__libcurl_config=$(STAGING_DIR)/usr/bin/curl-config
 BOINC_CONF_OPTS = \
 	--disable-apps \
 	--disable-boinczip \