build: Improve the build

build: Improve the build

* configure.ac: Mention dependencies as executable/library-specific
PKG_CHECK_MODULES() calls, instead of having one PKG_CHECK_MODULE()
call for each dependency and then listing those depdendencies'
CFLAGS and LIBS individually in each Makefile.am.
Also define the _VALAFLAGS in the same place, keeping things consistent.
* common.am: Clean this up, using more obvious variable names.
* various Makefile.am: Use the new structure.
Do not use AM_CFLAGS, instead specifying the CFLAGS, LIBS and VALAFLAGS
for each executable/library. As well as being cleaner, this would allow
us to convert to a non-recursive autotools build sometime in the future.

https://bugzilla.gnome.org/show_bug.cgi?id=689960