Add project's C args to test program

Normally the test program should not use the private config variables of
the library but we are temporarily adding them to make the tests succed.

The problem was reported in the github issue:

sile-typesetter#27

tests/meson.build

@@ -1,6 +1,13 @@
 libtexpdf_include = include_directories('..')
 
+# FIXME:
+# Above we should not add libtexpdf_cargs but if they are not added
+# compiling may fail because the header inclusion is conditioned to
+# defines like HAVE_CONFIG_H and others.
+# In addition the header files tries to conditionally include config.h
+# but this latter is private and not exported.
 test_create = executable('test-create', 'test-create.c',
+    c_args: libtexpdf_cargs,
     dependencies: libtexpdf_dep,
     include_directories: libtexpdf_include,
 )