add mixed appleclang/gfortran build

.travis.yml

@@ -230,6 +230,16 @@ matrix:
       env:
         - BTYPE="TARGET=NEHALEM BINARY=64 INTERFACE64=1 FC=gfortran-10"
     
+    - <<: *test-macos-clang
+      osx_image: xcode12
+      before_script:
+        - COMMON_FLAGS="NUM_THREADS=32"
+        - brew update
+        - brew install gcc@10
+      script:
+        - travis_wait 45 make QUIET_MAKE=1 $COMMON_FLAGS $BTYPE
+      env:
+        - BTYPE="TARGET=NEHALEM BINARY=64 FC=gfortran-10 CC=clang"
   #  - <<: *test-macos
   #    osx_image: xcode10
   #    env: