It is much easier if you use GNU Automake instead of writing your own
Makefiles. If you do that you do not have to worry about finding and
invoking the pkg-config script at all.
You can use PKG_CHECK_MODULES macro, or, libassuan also
provides an Automake macro that does all the work for you.
Check whether libassuan (at least version
minimum-version, if given) exists on the host system. If it is
found, execute action-if-found, otherwise do
action-if-not-found, if given.
Additionally, the function defines LIBASSUAN_CFLAGS to the
flags needed for compilation of the program to find the
assuan.h header file, and LIBASSUAN_LIBS to the linker
flags needed to link the program to the libassuan library.
You can use the defined Autoconf variables like this in your Makefile.am:
AM_CPPFLAGS = $(LIBASSUAN_CFLAGS) LDADD = $(LIBASSUAN_LIBS)