Fix check-* scripts when harfbuzz is a subproject

When harfbuzz is a subproject paths are in the form
"subprojects/harfbuzz/src/...". Instead of removing "src/" prefix, take
the absolute path and make it relative to current source dir.

This fix regression introduced in
https://github.com/harfbuzz/harfbuzz/pull/3394.

src/Makefile.am

diff --git a/src/Makefile.am b/src/Makefile.am
index 81eb4c9..e1b56a5 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -437,6 +437,7 @@
 
 TESTS_ENVIRONMENT = \
 	srcdir="$(srcdir)" \
+	base_srcdir="$(srcdir)" \
 	builddir="$(builddir)" \
 	MAKE="$(MAKE) $(AM_MAKEFLAGS)" \
 	HBSOURCES="$(HBSOURCES)" \