The LDFLAGS and CPPFLAGS variables contains command line flags that are passed to tools at various stages when compiling software. These should contain space-delimited flags.
export LDFLAGS=-L/some/directory
Later (note, you only have to export a variable once):
LDFLAGS="$LDFLAGS -L/other/directory"
Or,
LDFLAGS+=" -L/other/directory"
The PKG_CONFIG_PATH variable is used by the pkg-config tool. It's manual states (my emphasis):
PKG_CONFIG_PATH
This can be used to specify a colon-separated list of paths to search for package files. If given, this list of paths is prepended to the standard search path.
So,
export PKG_CONFIG_PATH=/some/directory
Later:
PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/other/directory
Or,
PKG_CONFIG_PATH+=:/other/directory