$ gcc -Wall -o test test.c

$ ls /usr/lib/pkgconfig/ | grep libxml
libxml-2.0.pc

$ cat /usr/lib/pkgconfig/libxml-2.0.pc
prefix=/usr
exec_prefix=/usr
libdir=/usr/lib
includedir=/usr/include
modules=1
Name: libXML
Version: 2.6.26
Description: libXML library version2.
Requires:
Libs: -L${libdir} -lxml2  -lz  -lm
Cflags: -I${includedir}/libxml2

$ pkg-config libxml-2.0 --cflags --libs
-I/usr/include/libxml2  -lxml2 -lz -lm

$ gcc -Wall `pkg-config libxml-2.0 --cflags --libs` -o test test.c