gdb: fix detection of compilation and linking flags for source-highlight

Currently there are two problems with the detection of
source-highlight via pkg-config in GDB's configure script:

1. The LDFLAGS variable is used to pass the 'pkg-config --libs' output
to AC_LINK_IFELSE, which results in the "-L/some/path
-lsource-highlight" preceding the conftest.cpp, which can result in a
failure to find symbols referenced in conftest.cpp, if the linker is
using --as-needed by default.

2. The CFLAGS variable is used to pass the 'pkg-config --cflags'
output to AC_LINK_IFELSE.  However, as the current language is C++,
AC_LINK_IFELSE will actuall use CXXFLAGS, not CFLAGS, so any flags
returned from pkg-config will not be seen.

This patch fixes both of these mistakes, allowing GDB to correctly
configure and build using source-highlight installed into a custom
prefix, e.g. ~/opt/gdb-git (because the system version of
source-highlight is too old).