Ignore RUBY_CONFIGURE_OPTS from environment in testing

Avoids failures caused by somebody's environment possibly having
RUBY_CONFIGURE_OPTS exported to have a value.

test/build.bats

@@ -5,6 +5,7 @@ export RUBY_BUILD_CACHE_PATH="$TMP/cache"
 export MAKE=make
 export MAKE_OPTS="-j 2"
 export CC=cc
+export -n RUBY_CONFIGURE_OPTS
 
 setup() {
   mkdir -p "$INSTALL_ROOT"

test/compiler.bats

@@ -5,6 +5,7 @@ export MAKE=make
 export MAKE_OPTS='-j 2'
 export -n CFLAGS
 export -n CC
+export -n RUBY_CONFIGURE_OPTS
 
 @test "require_gcc on OS X 10.9" {
   stub uname '-s : echo Darwin'