make install, closes #576
(manually cherry picked from commit 8d2b79a224c49109affa73d1af2165dc0acc0fc6)
Fixes to make install.

- fix DESTDIR for preprocessor and MEX files (it was duplicated)
- don't install libdynare++.a

(cherry picked from commit 9b5a09958c53371cea1e591ab36896ecd7483747)
Add more info on requirement on dsge_var prior weight needed for prior to be proper.

(cherry picked from commit 74fd0850d61f8ba464ff08ed42c4019d27722fb3)
(cherry picked from commit b69174477c1e0afbcc01ca8a1b63fa2014130418)
Document computation of theoretical mean at second order

(cherry picked from commit 7084913382dc892fa4385fb7783e5daec41449f7)
(cherry picked from commit fbdfaf640a0873fdb176fa8b62edde3025400b87)
Fix bug in DSGE-VAR. Initialization sample must be strictly larger than lag order

Also provides a more informative error message
Fix bug in imcforecast.m where covariance matrix was decomposed element-wise instead of via Cholesky.

(cherry picked from commit 04554967fc49539b570b7c266b274eb433783e32)
Deal with cases where too few draws are available for posterior moments

(cherry picked from commit bf674fa59fc300f6a5ba3aaf30eecfbff2521230)
Expand manual entry for conditional forecasts

(cherry picked from commit e7727ba2d3365cf5269ab718cfbed86c82243cbe)
Add description for conditional_forecasts

(cherry picked from commit b9aa971d734adebbb2a0b2ac16f787e838713c2d)