build-sys: Inject Automake's default CXXFLAGS to libdnf by default

Today I was trying to use gdb and noticed my libdnf build didn't
have debuginfo.  Now, I thought that's what `-DCMAKE_BUILD_TYPE=RelWithDebugInfo`
was doing but...I have no idea right now where I got that.

I looked at RPM builds, and the way this works is it exports CXXFLAGS.
Now for our C code, our defaults actually come from Autoconf.  Let's
do the beautiful hack of telling Autoconf we're going to use C++ so
it sets `CXXFLAGS` for us.

Closes: #1586
Approved by: jlebon

configure.ac

@@ -25,6 +25,10 @@ AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE
 
 AC_PROG_CC
+dnl we don't today use C++ in rpm-ostree, only in libdnf,
+dnl but this has the side effect of setting CXXFLAGS with -g
+dnl if not set, which we definitely want; cmake doesn't do that.
+AC_PROG_CXX
 AM_PROG_CC_C_O
 
 # dbus system dir
@@ -251,6 +255,7 @@ dnl I picked /usr/libexec/rpm-ostree just because we need an
 dnl arbitrary path - we don't actually install there.
 (set -euo pipefail; mkdir -p libdnf-build && cd libdnf-build &&
  set -x && 
+ export CXXFLAGS &&
  cmake \
      -DCMAKE_INSTALL_PREFIX:PATH=/usr/libexec/rpm-ostree \
      -DINCLUDE_INSTALL_DIR:PATH=/usr/libexec/rpm-ostree/include \