Respect system CFLAGS/CPPFLAGS/LDFLAGS

* only overwrite CFLAGS with intrusive "-g -O2"
  if they are empty
* always add non-intrusive warning flags
* use LIBS variable instead of LDFLAGS variable
* add CPPFLAGS and LDFLAGS where appropriate
Allow more control over install procedure

* allow use of DESTDIR which is standard in most build systems
* allow to specify separate INST_LIBDIR for multilib systems
* allow to control INST_INCLUDE and INST_DOCDIR as well
* quote variables where appropriate,
  make does not take care of that!
Merge pull request #9 from hasufell/master

small build system improvements