m4/nx-macros.m4: add NX_TARGET_USE_ELF macro, setting TARGET_ELF automake conditional if target compiler is generating ELF binaries.

m4/nx-macros.m4: Add NX_STRICT_OPTION and NX_DEFAULT_OPTIONS macros (borrowed from X.org's xorg-macros.m4).

nxcomp/configure.ac: Don't rely on pkg-config alone when it comes to testing for presence of libjpeg shared lib and header files.

 Especially systems still using the IJG libjpeg implementation
 are likely to lack the libjpeg.pc file.
 .
 So this adds some alternative way of detecting if all libjpeg related
 build-dependencies are in place.

nxcomp/configure.ac,m4/nx-macros.m4,nxcomp/src/Socket.h: Use AC_CHECK_TYPES to detect if type 'in_addr_t' is available.