Re-jig BUILD_C_CODE condition

* tta/configure.ac: Add conditional USE_SHARED_TXI_LIBRARIES to
be used when build targets depend on the C auxiliary libraries.
Require iconv to be found to set this.
(Suggestion from Patrice to make reason clearer for use of condition.)

Always set BUILD_C_CODE if any C code is being built.

* tta/Makefile.am: Don't build in C or SWIG directories if
USE_SHARED_TXI_LIBRARIES is false.

* tta/perl/XSTexinfo/Makefile.am: use BUILD_C_CODE condition around
building XSParagraph.la and MiscXS.la so that they are built under the
same condition as that of gnulib upon which they depend.

This mismatch led to a build failure on macOS 26, where iconv was not
found and the other XS modules were not built.  Report from Bruno.

Use USE_SHARED_TXI_LIBRARIES instead of BUILD_C_CODE for other
*.la targets.