bpo-46602: Do not append conftest.c (GH-31062)

The heredoc creation statements use >> to append conftest.c.  This can cause
tricky build issues if the file is not correctly removed prior to its
name being reused (such name is reused several times for different
contextual tests during the build).  One such result from appending may
cause #include <ac_nonexistent.h> to persist when testing to acquire
PLATFORM_TRIPLET.  This can then lead to downstream issues concerning SOABI.

Author: adanhawth

Misc/NEWS.d/next/Build/2022-02-02-02-06-07.bpo-46602.8GaOZ2.rst

@@ -0,0 +1 @@
+Tidied up configure.ac so that conftest.c is truncated rather than appended.  This assists in the case where the 'rm' of conftest.c fails to happen between tests.  Downstream issues such as a clobbered SOABI can result.

configure

@@ -833,7 +833,7 @@ fi
 
 
 AC_MSG_CHECKING([for the platform triplet based on compiler characteristics])
-cat >> conftest.c <<EOF
+cat > conftest.c <<EOF
 #undef bfin
 #undef cris
 #undef fr30
@@ -1028,7 +1028,7 @@ AC_CACHE_CHECK([for -Wl,--no-as-needed], [ac_cv_wl_no_as_needed], [
 AC_SUBST(NO_AS_NEEDED)
 
 AC_MSG_CHECKING([for the Android API level])
-cat >> conftest.c <<EOF
+cat > conftest.c <<EOF
 #ifdef __ANDROID__
 android_api = __ANDROID_API__
 arm_arch = __ARM_ARCH