No need to specify EXTRA_DIST

We build a tarball (flowgrind release) not by calling 'make dist'. No
need to specify EXTRA_DIST
Use 'maintainer-clean' rule for deleting files

While building flowgrind tarball delete generated files w/ 'make
maintainer-clean' instead of list them all explicitly
Use .PHONY instead of FORCE target

For rules without recipes or prerequisites, GNU recommends to use
.PHONY, which is more explicit and more efficient. See
https://www.gnu.org/software/make/manual/html_node/Force-Targets.html
Use AC_PATH_TOOL instead of AC_PATH_PROG

Needed for cross-compiling
Makefile rule for doxygen

- add a autoconf check for doxygen
- introduce new make rule for doxygen (make html)
[Coding Style] Force line breaks in Makefile.am
[Coding Style] increase readability of configure.ac

- force line breaks
- group command logically together
- add comments to group of commands
- fix indention
- use squared brackets for function parameters
Use AC_SUBST instead of mangle LDFLAGS and CFLAGS directly
Preprocessor needs CPPFLAGS to find includes

On FreeBSD and OS X preprocessor cannot find libgsl. This patch will fix
./configure waring "accepted by the compiler, rejected by the preprocessor"
Improve checking for --enable-debug in configure.ac

- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for libpcap in configure.ac

- use AC_ARG_WITH instead of AC_ARG_ENABLE. BCP when checking for
  external library
- exit ./configure w/ an error if user request libpcap but library isn't
  found
- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for libgsl in configure.ac

- use AC_ARG_WITH instead of AC_ARG_ENABLE. BCP when checking for
  external library
- exit ./configure w/ an error if user request libgsl but library isn't
  found
- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for CURL in configure.ac

- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Improve checking for xmlrpc-c in configure.ac

- use AS_IF instead of bash code
- force line breaks
- fix indention
- use squared brackets for function parameters
Treat pcap and gsl as optional packages
[Coding Style] bracketing leads to empty lines
We do not use make for making distributions
Merge remote-tracking branch 'upstream/next' into next
Typo in configure.ac
Merge pull request #117 from alexander-zimmermann/next

Improve autotool magic for multi OS support