Turn some compiler warnings into errors

Re-apply the reverted commit 98bc7fa50d, this
time with a clear error message in case autoconf-archive is not installed.

Change-Id: I631bde22e79ec0318eb9c8114db5861a1d635816
Tweaked-by: Neels Hofmeyr <nhofmeyr@sysmocom.de>

debian/control

@@ -4,6 +4,7 @@ Section: net
 Priority: optional
 Build-Depends: debhelper (>= 9),
                autotools-dev,
+               autoconf-archive,
                pkg-config,
                libgtp0-dev,
                libosmocore-dev,

openbsc/configure.ac

@@ -25,6 +25,11 @@ if test "x$PKG_CONFIG_INSTALLED" = "xno"; then
 fi
 PKG_PROG_PKG_CONFIG([0.20])
 
+dnl check for AX_CHECK_COMPILE_FLAG
+m4_ifdef([AX_CHECK_COMPILE_FLAG], [], [
+	AC_MSG_ERROR([Please install autoconf-archive; re-run 'autoreconf -fi' for it to take effect.])
+	])
+
 dnl checks for libraries
 AC_SEARCH_LIBS([dlopen], [dl dld], [LIBRARY_DL="$LIBS";LIBS=""])
 AC_SUBST(LIBRARY_DL)
@@ -140,6 +145,13 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([char foo;])],
 CFLAGS="$saved_CFLAGS"
 AC_SUBST(SYMBOL_VISIBILITY)
 
+AX_CHECK_COMPILE_FLAG([-Werror=implicit], [CFLAGS="$CFLAGS -Werror=implicit"])
+AX_CHECK_COMPILE_FLAG([-Werror=maybe-uninitialized], [CFLAGS="$CFLAGS -Werror=maybe-uninitialized"])
+AX_CHECK_COMPILE_FLAG([-Werror=memset-transposed-args], [CFLAGS="$CFLAGS -Werror=memset-transposed-args"])
+AX_CHECK_COMPILE_FLAG([-Werror=null-dereference], [CFLAGS="$CFLAGS -Werror=null-dereference"])
+AX_CHECK_COMPILE_FLAG([-Werror=sizeof-array-argument], [CFLAGS="$CFLAGS -Werror=sizeof-array-argument"])
+AX_CHECK_COMPILE_FLAG([-Werror=sizeof-pointer-memaccess], [CFLAGS="$CFLAGS -Werror=sizeof-pointer-memaccess"])
+
 # Coverage build taken from WebKit's configure.in
 AC_MSG_CHECKING([whether to enable code coverage support])
 AC_ARG_ENABLE(coverage,