SRCDIR_<dir> variable was introduced
SRC_PATHADD variable was introduced
Variable MKC_SOURCE_DIR was introduced.
Documentation for OPSYS_TARGET, TOOLDIR and MACHINE_GNU_PLATFORM
${SRCTOP}/Makefile.common is included by all subprojects if it exists
TODO: -1 task (done)
mkc_imp.pkg-config.mk: new vars MKC_CHECK_PKGCONFIG and MKC_REQUIRE_PKGCONFIG.
They work just like other MKC_{CHECK,REQUIRE}_* variables
from mkc.configure.mk. Thanks to Andrew Shadura for pushing me
to this direction. See manual page for details.