try to fix Travis MacOS issue

.travis.yml

@@ -64,9 +64,9 @@ matrix:
           - zip
           - qt5-default
     - os: osx
-      osx_image: xcode9.4.1 # we target latest MacOS Mojave
+      osx_image: xcode10.2 # we target latest MacOS Mojave
       sudo: true
-      compiler: gcc-6 # Use clang instead of gcc in MacOS
+      compiler: gcc-4.9 # Use clang instead of gcc in MacOS
       addons:
         homebrew:
           packages:
@@ -77,6 +77,7 @@ matrix:
           - fontconfig
           - git
           - gcc@6
+          - gcc@4.9
           - gawk
           - icarus-verilog
           - libxml++

.travis/common.sh

@@ -31,12 +31,12 @@ function end_section() {
 
 # For Mac OS, we use g++ and gcc as default compilers
 if [[ $TRAVIS_OS_NAME == 'osx' ]]; then
-  export CC=gcc-6
-  export CXX=g++-6
+  export CC=gcc-4.9
+  export CXX=g++-4.9
   # export PATH="/usr/local/opt/bison/bin:/usr/local/bin:$PATH"
   # export PATH="/usr/local/opt/qt/bin:$PATH"
   # Install header files in Mojave, if not gcc-4.9 cannot spot stdio.h
-  #sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / -allowUntrusted
+  sudo installer -pkg /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg -target / -allowUntrusted
 else
   # For linux, we use g++-8 and gcc-8 as default compilers
   export CC=gcc-8