unix: improve ncurses cross-compiling

Previously, cross-compiles only worked on Apple and x86_64 -> i686.
Upcoming commits to introduce ARM Linux cross-compiles uncovered more
issues.

This commit contains changes to work around those.

We have to save and restore CC to get the host build to use the
correct compiler.

We also need to munge the path to the host compiler to use the host
GCC present in the new cross-compile environment.

And we need to disable application stripping, since `strip` doesn't
recognize the target. This is probably the hackiest change. But I
couldn't figure out (only spent minimal effort though) on how to
get ncurses's build system to use the correct `strip` binary.

Author: indygreg

cpython-unix/build-ncurses.sh

@@ -19,11 +19,16 @@ tar -xf ncurses-${NCURSES_VERSION}.tar.gz
 if [[ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
   echo "building host ncurses to provide modern tic for cross-compile"
 
+  OLD_CC=${CC}
+  unset CC
+
   pushd ncurses-${NCURSES_VERSION}
   ./configure --prefix=${TOOLS_PATH}/host --without-cxx --without-tests --without-manpages --enable-widec
   make -j ${NUM_CPUS}
   make -j ${NUM_CPUS} install
 
+  export CC=${OLD_CC}
+
   popd
 
   # Nuke and re-pave the source directory.
@@ -33,18 +38,32 @@ fi
 
 pushd ncurses-${NCURSES_VERSION}
 
+# `make install` will strip installed programs (like tic) by default. This is
+# fine. However, cross-compiles can run into issues where `strip` doesn't
+# recognize the target architecture. We could fix this by overriding strip.
+# But we don't care about the installed binaries, so we simply disable
+# stripping of the binaries.
 CONFIGURE_FLAGS="
     --build=${BUILD_TRIPLE}
     --host=${TARGET_TRIPLE}
     --prefix=/tools/deps
     --without-cxx
     --without-tests
     --without-manpages
+    --disable-stripping
     --enable-widec"
 
-# ncurses wants --with-build-cc when cross-compiling.
+# ncurses wants --with-build-cc when cross-compiling. But it insists on CC
+# and this value not being equal, even though using the same binary with
+# different compiler flags is doable!
 if [[ "${BUILD_TRIPLE}" != "${TARGET_TRIPLE}" && "${PYBUILD_PLATFORM}" != "macos" ]]; then
-  CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-cc=${TOOLS_PATH}/${TOOLCHAIN}/bin/clang"
+  # Look for and use our Clang toolchain by default. If not present, fall
+  # back to likely path to system GCC.
+  if [ -e "${TOOLS_PATH}/${TOOLCHAIN}/bin/clang" ]; then
+    CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-cc=${TOOLS_PATH}/${TOOLCHAIN}/bin/clang"
+  else
+    CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-cc=/usr/bin/x86_64-linux-gnu-gcc"
+  fi
 fi
 
 if [ "${PYBUILD_PLATFORM}" = "macos" ]; then