introduce "musthave" files, so tests can easily check for HAVE_* prer…

…equisites from config.h

docs/src/code/writing-tests.adoc

@@ -79,6 +79,11 @@ checkresult::
    At the moment, the use of _sudo_ can be flagged, and tests requiring sudo can be skipped when using `runtests -u`.
    To flag such requirements, add a line with `Restrictions: sudo` to this file.
 
+ musthave::
+   This file can contain a list of prerequisites from config.h (one per line). If it's not met, the test will be skipped.
+   e.g. if your test depends on config.h having `#define HAVE_TK_H 1` and `#define HAVE_LIBMODBUS3 yes`, add a line with
+   `TK_H` and a line with `LIBMODBUS3` to this file.
+
 == Some testing approaches
 
 There are various ways to structure a test, depending on what one wants to test.

scripts/runtests.in

@@ -126,6 +126,18 @@ run_tests () {
 
     while read testname; do
 	testdir=$(dirname $testname)
+        # check if there's a "musthave" file with prerequisites from config.h
+        if [ -e $testdir/musthave ] ; then
+            # one prerequisite per line
+            while IFS= read -r prereq ; do
+                if ! grep --quiet --regexp "^#define HAVE_$prereq.*" "$TOPDIR/src/config.h" ; then
+                    echo "Skipping test for missing prerequisite \"$prereq\": $testdir" 1>&2
+                    SKIP=$(($SKIP+1))
+                    continue 3
+                fi
+            done < $testdir/musthave
+        fi
+        # skip test if there's a "skip" file
 	if [ -e $testdir/skip ]; then
 	    if ! [ -x $testdir/skip ] || ! $testdir/skip; then
 		echo "Skipping disabled test: $testdir" 1>&2