Revert "Turn some warnings into errors"

This introduces a failure with ./configure on my machine:

  ../configure: line 6612: syntax error near unexpected token `-Werror=implicit,'
  ../configure: line 6612: `AX_CHECK_COMPILE_FLAG(-Werror=implicit, CFLAGS="$CFLAGS -Werror=implicit")'
  Makefile:420: recipe for target 'config.status' failed
  make[1]: *** [config.status] Error 2

Installing autoconf-archive did not fix the error.
So even though jenkins seems to accept this, I cannot build with this patch.

Let's find out how to do this in a way that all systems can still build
and then re-apply this patch.

This reverts commit fd161ccce8.

Change-Id: I2b368500b07f233882ef854eca1fa21f6df39e29

debian/control

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

openbsc/configure.ac

@@ -140,14 +140,6 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([char foo;])],
 CFLAGS="$saved_CFLAGS"
 AC_SUBST(SYMBOL_VISIBILITY)
 
-# FIXME: use CC_CHECK_CFLAG_APPEND([-Werror=implicit])
-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,