Really make -Werror conditional to BUILD_WERROR

Otherwise it will fail with an error message like this one:

elf_getarsym.c:290:9: error: 'n' may be used uninitialized in this
function [-Werror=maybe-uninitialized]
    arsym[n].as_name = NULL;
	 ^
cc1: all warnings being treated as errors

[Vincent: tweak patch for 0.166]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>

diff -rup a/config/eu.am b/config/eu.am
--- a/config/eu.am	2016-03-02 16:25:38.000000000 +0000
+++ b/config/eu.am	2016-06-17 15:05:08.270974835 +0100
@@ -65,7 +65,6 @@ AM_CFLAGS = -std=gnu99 -Wall -Wshadow -W
 	    -Wold-style-definition -Wstrict-prototypes \
 	    $(LOGICAL_OP_WARNING) $(DUPLICATED_COND_WARNING) \
 	    $(NULL_DEREFERENCE_WARNING) \
-	    $(if $($(*F)_no_Werror),,-Werror) \
 	    $(if $($(*F)_no_Wunused),,-Wunused -Wextra) \
 	    $(if $($(*F)_no_Wstack_usage),,$(STACK_USAGE_WARNING)) \
 	    $($(*F)_CFLAGS)