CF3_WITH_LIBRARY now takes into account previous *FLAGS

It is used for libxml2, which by defaults exists in a libxml2/
subdirectory inside include/. Thus, before calling the CF3_WITH_LIBRARY
macro, we set LIBXML2_CPPFLAGS according to the --with-libxml2 argument
together with the subdirectory prefix, and the macro is happily
detecting libxml2 like that.
(cherry picked from commit 52adea90dfa456f8d599ec092a3133cc2b1f7363)

Conflicts resolved in:
	configure.ac
	m4/cf3_with_library.m4
Add missing quotes in shell expression to fix AIX build.
(cherry picked from commit 1876a215a2740480f15ddfbf10f381e75aaea126)
CPPFLAGS now only include libutils directory. Yay!

So the last offenders modifying the environment CPPFLAGS were MYSQL,
POSTGRESQL, LIBXML2. They have been converted to have their own set of
blah_CPPFLAGS.

In addition, libxml2 includes were removed from within cf3.defs.h and
added to the source files that really need them.

As a result, cf-upgrade compilation line is at last free of dependencies:

gcc -DHAVE_CONFIG_H -I. -I../libutils   -std=gnu99  -pthread -g -O2 -g -Wall -Wno-pointer-sign -Werror=implicit-function-declaration -Wunused-parameter -g3 -O0  -MT cf-upgrade.o -MD -MP -MF .deps/cf-upgrade.Tpo -c -o cf-upgrade.o cf-upgrade.c
(cherry picked from commit e15618b62f658a4ceaacd0b6615bf7cd6ab38c4a)

Conflicts resolved in:
	cf-agent/files_edit.h
	configure.ac
	libpromises/cf3.defs.h
patch configure to correctly find libxml2
(cherry picked from commit 7b2c2521f79bb680afb64af07759b1096a9aa4a8)

Conflicts resolved in: configure.ac