Use environment variables to control
how libsass is linked:

LIBSASS_EXT variable empty or unset

Build a bundled libsass source
(from src/libsass).
This is the default.

LIBSASS_EXT=auto

libsass is located using pkg-config
and linked dynamically

LIBSASS_EXT=yes
LIBSASS_CFLAGS=    compiler flags
LIBSASS_LDFLAGS=   linker flags
LIBSASS_LIBRARY=   library to link

Link libsass manually, by adding
compiler and linker flags.

For example this way libsass can
be linked in statically (on Unix):

LIBSASS_EXT=yes
LIBSASS_CFLAGS=-I/usr/local/include
LIBSASS_LIBRARY=/usr/local/lib/libsass.a

When using LIBSASS_EXT there is no need
to checkout the libsass source into src/libsass submodule
and the library from the package management
system can be used.

PR: #139
PR: #389
PR: #744