Disable GUI parts build for PPC and S/390 Travis platforms

People are unlikely to use wxGTK on those anyhow, and it takes an
awfully long time to build, especially on PPC.

.travis.yml

@@ -67,11 +67,11 @@ matrix:
           name: wxGTK ARM64
         - os: linux
           arch: ppc64le
-          env: wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
+          env: wxCONFIGURE_FLAGS="--disable-sys-libs --disable-gui" wxLXC=1
           name: wxGTK PowerPC64
         - os: linux
           arch: s390x
-          env: wxCONFIGURE_FLAGS="--disable-sys-libs" wxLXC=1
+          env: wxCONFIGURE_FLAGS="--disable-sys-libs --disable-gui" wxLXC=1
           name: wxGTK S/390
 
 branches:

build/tools/before_install.sh

@@ -16,6 +16,7 @@ case $(uname -s) in
                 *--with-motif*)    libtoolkit_dev='libmotif-dev libxmu-dev' ;;
                 *--with-qt*)       libtoolkit_dev='qtdeclarative5-dev libqt5opengl5-dev';;
                 *--with-x11*)      extra_deps='libpango1.0-dev' ;;
+                *--disable-gui*)   ;;
                 *)
                     case "$wxGTK_VERSION" in
                         3)  libtoolkit_dev=libgtk-3-dev