cmark-gfm: Make sure CMARK_GFM_STATIC_DEFINE is set to 0 or 1

[steffen-kiess]

Currently (with meson 1.0.1) the file build/subprojects/cmark-gfm-0.29.0.gfm.13/cmark-gfm_export.h will contain this (if default_library=static is set):

/* Is this an exclusively static build */
#if True && ! defined CMARK_GFM_STATIC_DEFINE
#  define CMARK_GFM_STATIC_DEFINE
#endif

This will cause a warning with -Wundef and will not work, because True will be evaulated as 0 by the preprocessor:

subprojects/cmark-gfm-0.29.0.gfm.13/cmark-gfm_export.h:5:5: warning: "True" is not defined, evaluates to 0 [-Wundef]
    5 | #if True && ! defined CMARK_GFM_STATIC_DEFINE

This PR makes sure that @CMARK_GFM_STATIC_DEFINE@ is always set to 0 or 1.