Travis-CI: ensure that CFLAGS is used for configure

Without this, clang-5.0 fails to detect that we have wcslen(), which leads to a
conflict with the `#ifndef HAVE_WCSLEN` version in libarchive.

.travis.yml

@@ -10,15 +10,15 @@ script:
       tar/tarsnap-tree.o tar/tarsnap-util.o tar/tarsnap-write.o
       libarchive/libarchive.a
   && make "CFLAGS=$CFLAGS $CFLAGS_STRICT"
+env:
+  - CFLAGS="-std=c99 -O2 -Wall -Wextra -Werror"
 matrix:
   include:
     - compiler: gcc
-      env: CFLAGS="-std=c99 -O2 -Wall -Wextra -Werror"
-           CFLAGS_STRICT="-Wpedantic -pedantic-errors -Wno-clobbered"
+      env: CFLAGS_STRICT="-Wpedantic -pedantic-errors -Wno-clobbered"
            CFLAGS_LIBARCHIVE="$CFLAGS_STRICT"
     - compiler: clang
-      env: CFLAGS="-std=c99 -O2 -Wall -Wextra -Werror"
-           CFLAGS_STRICT="-Weverything -Werror -Wno-#warnings -Wno-pedantic -Wno-padded -Wno-format-nonliteral -Wno-disabled-macro-expansion -Wno-undef -Wno-documentation-unknown-command -Wno-missing-noreturn -Wno-unused-function -Wno-reserved-id-macro -Wno-cast-align"
+      env: CFLAGS_STRICT="-Weverything -Werror -Wno-#warnings -Wno-pedantic -Wno-padded -Wno-format-nonliteral -Wno-disabled-macro-expansion -Wno-undef -Wno-documentation-unknown-command -Wno-missing-noreturn -Wno-unused-function -Wno-reserved-id-macro -Wno-cast-align"
            CFLAGS_LIBARCHIVE="-Weverything -Werror -Wno-#warnings -Wno-pedantic -Wno-padded -Wno-format-nonliteral -Wno-disabled-macro-expansion -Wno-missing-noreturn -Wno-sign-conversion -Wno-undef -Wno-shorten-64-to-32 -Wno-shadow -Wno-conversion -Wno-unused-macros -Wno-class-varargs -Wno-unreachable-code -Wno-tautological-overlap-compare -Wno-float-equal -Wno-unreachable-code-break -Wno-unused-function -Wno-reserved-id-macro -Wno-comma"
 addons:
   apt: