Travis: Build and test also on Alpine Linux (musl libc)

Alpine jobs needs sudo (for chroot), so they run on VMs infrastructure.
That's why they are much slower than other jobs.

.travis.yml

@@ -13,9 +13,10 @@ jobs:
         apt:
           packages:
             - gfortran
+      before_script: &common-before
+        - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=NEHALEM NUM_THREADS=32"
       script:
         - set -e
-        - COMMON_FLAGS="DYNAMIC_ARCH=1 TARGET=NEHALEM NUM_THREADS=32"
         - make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
         - make -C test $COMMON_FLAGS $BTYPE
         - make -C ctest $COMMON_FLAGS $BTYPE
@@ -51,12 +52,58 @@ jobs:
             - binutils-mingw-w64-x86-64
             - gcc-mingw-w64-x86-64
             - gfortran-mingw-w64-x86-64
+      before_script: *common-before
       script:
-        - make QUIET_MAKE=1 DYNAMIC_ARCH=1 TARGET=NEHALEM NUM_THREADS=32 $BTYPE
+        - make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
       env:
         - TARGET_BOX=WIN64
         - BTYPE="BINARY=64 HOSTCC=gcc CC=x86_64-w64-mingw32-gcc FC=x86_64-w64-mingw32-gfortran"
 
+    # Build & test on Alpine Linux inside chroot, i.e. on system with musl libc.
+    # These jobs needs sudo, so Travis runs them on VM-based infrastructure
+    # which is slower than container-based infrastructure used for jobs
+    # that don't require sudo.
+    - &test-alpine
+      stage: test
+      dist: trusty
+      sudo: true
+      language: minimal
+      before_install:
+        - "wget 'https://raw.githubusercontent.com/alpinelinux/alpine-chroot-install/v0.6.0/alpine-chroot-install' \
+              && echo 'a827a4ba3d0817e7c88bae17fe34e50204983d1e  alpine-chroot-install' | sha1sum -c || exit 1"
+        - alpine() { /alpine/enter-chroot -u "$USER" "$@"; }
+      install:
+        - sudo sh alpine-chroot-install -p 'build-base gfortran perl linux-headers'
+      before_script: *common-before
+      script:
+        - set -e
+        - alpine make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
+        - alpine make -C test $COMMON_FLAGS $BTYPE
+        - alpine make -C ctest $COMMON_FLAGS $BTYPE
+        - alpine make -C utest $COMMON_FLAGS $BTYPE
+      env:
+        - TARGET_BOX=LINUX64_MUSL
+        - BTYPE="BINARY=64"
+
+    - <<: *test-alpine
+      env:
+        - TARGET_BOX=LINUX64_MUSL
+        - BTYPE="BINARY=64 USE_OPENMP=1"
+
+    - <<: *test-alpine
+      env:
+        - TARGET_BOX=LINUX64_MUSL
+        - BTYPE="BINARY=64 INTERFACE64=1"
+
+    # Build with the same flags as Alpine do in OpenBLAS package.
+    - <<: *test-alpine
+      env:
+        - TARGET_BOX=LINUX64_MUSL
+        - BTYPE="BINARY=64 NO_AFFINITY=1 USE_OPENMP=0 NO_LAPACK=0 TARGET=core2"
+
+  allow_failures:
+    - <<: *test-alpine-openmp
+
 # whitelist
 branches:
   only: