build: Modernize build system a bit
Stop using GNOME_COMMON_INIT as it is deprecated. Do not use AM_MAINTAINER_MODE as it is not recommended by automake developers. Use LT_PREREQ & LT_INIT for libtool checks. Require intltool 0.40.6 to avoid various bugs. Do not use AC_ISC_POSIX, AC_HEADER_STDC, AM_PROG_CC_C_O as they shouldn't be needed on modern systems. Remove checks for perl and awk, and the useless --disable-rebuilds configure flag.