DOC: install: describe the DEP variable
The variable was quickly mentioned in the makefile but not in the INSTALL
file. Let's describe its use cases and limitations.
diff --git a/INSTALL b/INSTALL
index 83d5631..bdcf7d5 100644
--- a/INSTALL
+++ b/INSTALL
@@ -529,6 +529,16 @@
recommended way to build when developing, and it is expected that contributed
patches were tested with ERR=1.
+The DEP variable is automatically set to the list of include files and also
+designates a file that contains the last build options used. It is used during
+the build process to compute dependencies and decide whether or not to rebuild
+everything (we do rebuild everything when .h files are touched or when build
+options change). Sometimes when performing fast build iterations on inline
+functions it may be desirable to avoid a full rebuild. Forcing this variable
+to be empty will be sufficient to achieve this. This variable must never be
+forced to produce final binaries, and must not be used during bisect sessions,
+as it will often lead to the wrong commit.
+
If you need to pass other defines, includes, libraries, etc... then please
check the Makefile to see which ones will be available in your case, and
use/override the USE_* variables from the Makefile.