Use an explicit matrix instead, avoid bad clang combinations etc

.travis.yml

 language: c
-os:
-  - linux
-  - osx
-
-env:
-  matrix:
-    - BUNDLEDLIBTOM=--disable-bundled-libtom WEXTRAFLAGS=-Werror
-    - BUNDLEDLIBTOM=--enable-bundled-libtom
-    - MULTI=1
-    - NOWRITEV=1
+git:
+  depth: 3
 matrix:
-  exclude:
-    # TODO: remove this when libtomcrypt has been updated by ubuntu/homebrew.
-    # https://github.com/libtom/libtomcrypt/issues/82
-    - compiler: clang
-      env: BUNDLEDLIBTOM=--disable-bundled-libtom WEXTRAFLAGS=-Werror
-
-compiler:
-  - gcc
-  - clang
+  include:
+    # subsequent matrix options use these first settings
+    - os: linux
+      compiler: gcc
+      env: WEXTRAFLAGS=-Werror
+    - env: MULTI=1 WEXTRAFLAGS=-Werror
+    # libtom has some warnings, so no WEXTRAFLAGS
+    - env: BUNDLEDLIBTOM=--enable-bundled-libtom WEXTRAFLAGS=""
+    - env: NOWRITEV=1 WEXTRAFLAGS=-Werror
+    # libtomcrypt 1.18.1 fixes clang problems, distro doesn't have that yet
+    - os: linux
+      compiler: clang
+      env: BUNDLEDLIBTOM=--enable-bundled-libtom WEXTRAFLAGS=""
+    - os: osx
+      compiler: clang
+      env: WEXTRAFLAGS=""
 # container-based builds
 sudo: false
@@ -36,9 +35,6 @@ addons:
 before_install:
   - if [ "$CC" = "clang" ]; then WEXTRAFLAGS="$WEXTRAFLAGS -Wno-error=incompatible-library-redeclaration" ; fi  # workaround
-install:
-  - if [ "$TRAVIS_OS_NAME" = "osx" -a "$BUNDLEDLIBTOM" = "--disable-bundled-libtom" ]; then brew update > /dev/null && brew install libtomcrypt libtommath ; fi
-
 script:
   - autoconf && autoheader && ./configure "$BUNDLEDLIBTOM" CFLAGS="-O2 -Wall -Wno-pointer-sign $WEXTRAFLAGS" --prefix="$HOME/inst"
   - if [ "$NOWRITEV" = "1" ]; then sed -i -e s/HAVE_WRITEV/DONT_HAVE_WRITEV/ config.h ; fi