Issue 27453: $CPP invocation in configure must use $CPPFLAGS

Motivation: Android NDK is deprecating GCC and moving ot Clang/LLVM. From [1]:

We strongly recommend switching to Clang.
GCC in the NDK is now deprecated in favor of Clang.

This patch fixes the only one problem when migrating GCC to Clang.

In my build script, $CPP is set to "${ANDROID_NDK}/toolchains/llvm/prebuilt/linux-x86_64/bin/clang -E" and $CPPFLAGS is set to "-target armv7-none-linux-androideabi -gcc-toolchain ${ANDROID_NDK}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64 --sysroot ${ANDROID_NDK}/platforms/android-21/arch-arm/usr -I${PREFIX}/include -DANDROID". I'm not sure whether it's the correct usage of clang, but at least it works.

With such a configuration, most Python parts builds well. Just something wrong - extensions as built as xyz.cpython-36m-x86_64-linux-gnu.so. My patch just fixes $(PLATFORM_TRIPLET) detection.

[1] https://developer.android.com/ndk/downloads/revision_history.html

With configure run as './configure CC="$(CC)" ...' and when CC is set to:
CC = clang --sysroot=$(SYSROOT) -target $(TARGET) -gcc-toolchain $(GCC_TOOLCHAIN)

and building for armv7 on android-21, then configure runs $CPP as:

checking for CPP... clang --sysroot=/opt/android-ndk/platforms/android-21/arch-arm -target armv7-none-linux-androideabi -gcc-toolchain /opt/android-ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64 -E

as can be seen by inserting in configure.ac:
AC_MSG_CHECKING([for CPP])
AC_MSG_RESULT([$CPP])

and the shared libraries are named 'module_name.cpython-36m-arm-linux-gnueabi.so'.

OTOH, the 'configure' script runs its own preprocessings with the following idiom:
if (eval "$ac_cpp conftest.$ac_ext") 2>&5 | $EGREP "memchr" >/dev/null 2>&1; then :

where 'ac_cpp' is set to ac_cpp='$CPP $CPPFLAGS'.
So the proposed patch LGTM.

PLATFORM_TRIPLETs are specific to multiarch systems such as debian and should not be relied upon on Android, but they should not be entirely wrong of course.

Several days ago I have once tried to set
CC="clang -target $(TARGET) -gcc-toolchain $(GCC_TOOLCHAIN)", and it failed to build some dependency of Python (I can't remember). Just tried  and everything is OK. Dunno what's changed. Anyway both approaches (-target in $CC/$CXX/$CPP and -target in $CFLAGS/$CXXFLAGS/$CPPFLAGS) should be supported.

ndk-clang-preprocessor.patch LGTM.

New changeset 4cb94e561e2d by Xavier de Gaye in branch '3.5':
Issue #27453: CPP invocation in configure must use CPPFLAGS.
https://hg.python.org/cpython/rev/4cb94e561e2d

New changeset 92b3ce7ca95c by Xavier de Gaye in branch 'default':
(merge from 3.5) Issue #27453: CPP invocation in configure must use CPPFLAGS.
https://hg.python.org/cpython/rev/92b3ce7ca95c

Thanks for the patch Chi Hsuan Yen!

stage: commit review -> resolved

stage: commit review