Makefile.am: use CXXFLAGS instead of CFLAGS for protoc-c/protoc-c, t/generated_code2_cxx_generate_packed_data

these are C++ programs, not C programs, so use CXXFLAGS instead of
CFLAGS.

based on a patch from Andrei Nigmatulin.
configure.ac: use $protobuf_CFLAGS to find protobuf header files

the protobuf header files may be installed in a non-standard location
and thus we need to use the CFLAGS registered for protobuf in pkg-config
in order to find them.

based on a patch from Andrei Nigmatulin.
configure.ac: search the pkg-config exec_prefix for the protobuf compiler

protoc may not be on the default PATH, so augment $PATH with the
executable path registered by pkg-config for the protobuf package.

additionally declare PROTOC as a precious variable, thus allowing it to
be explicitly set by the user at ./configure time.

based on a patch from Andrei Nigmatulin.