libtextstyle: Fix compilation error on Android.
* libtextstyle/gnulib-local/m4/libglib.m4 (gl_LIBGLIB): Test whether struct
lconv has a 'decimal_point' field.
* libtextstyle/gnulib-local/lib/glib/gstrfuncs.c (g_ascii_strtod,
g_ascii_formatd): If struct lconv does not have a 'decimal_point' field, use "."
instead.
