Replace H5detect's build-time detection of C99 integer properties with a

table-driven routine, `H5T__init_native_int()`, that is run at library
initialization time.

Always respect the alignment used by the compiler for integers.  The
library invites trouble by using different alignment than the compiler
expects.  Here and there update a comment about alignment.

Retire the H5detect code that tries to find the least permissible
integer alignment by running experiments and catching any signals thrown
or unexpected results.

src/H5T.c

@@ -473,35 +473,21 @@ hid_t H5T_NATIVE_INT_FAST64_g   = FAIL;
 hid_t H5T_NATIVE_UINT_FAST64_g  = FAIL;
 
 /*
- * Alignment constraints for native types. These are initialized at run time
- * in H5Tinit.c.  These alignments are mainly for offsets in HDF5 compound
- * datatype or C structures, which are different from the alignments for memory
- * address below this group of variables.
+ * Alignment constraints for HDF5 types.  Accessing objects of these
+ * types with improper alignment invokes C undefined behavior, so the
+ * library lays out objects with correct alignment, always.
+ *
+ * A value of N indicates that the data must be aligned on an address
+ * ADDR such that 0 == ADDR mod N. When N=1 no alignment is required;
+ * N=0 implies that alignment constraints were not calculated.  These
+ * values are used for structure alignment.
  */
-size_t H5T_NATIVE_SCHAR_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_UCHAR_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_SHORT_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_USHORT_COMP_ALIGN_g  = 0;
-size_t H5T_NATIVE_INT_COMP_ALIGN_g     = 0;
-size_t H5T_NATIVE_UINT_COMP_ALIGN_g    = 0;
-size_t H5T_NATIVE_LONG_COMP_ALIGN_g    = 0;
-size_t H5T_NATIVE_ULONG_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_LLONG_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_ULLONG_COMP_ALIGN_g  = 0;
-size_t H5T_NATIVE_FLOAT_COMP_ALIGN_g   = 0;
-size_t H5T_NATIVE_DOUBLE_COMP_ALIGN_g  = 0;
-size_t H5T_NATIVE_LDOUBLE_COMP_ALIGN_g = 0;
-
-size_t H5T_POINTER_COMP_ALIGN_g     = 0;
-size_t H5T_HVL_COMP_ALIGN_g         = 0;
-size_t H5T_HOBJREF_COMP_ALIGN_g     = 0;
-size_t H5T_HDSETREGREF_COMP_ALIGN_g = 0;
-size_t H5T_REF_COMP_ALIGN_g         = 0;
+size_t H5T_POINTER_ALIGN_g     = 0;
+size_t H5T_HVL_ALIGN_g         = 0;
+size_t H5T_HOBJREF_ALIGN_g     = 0;
+size_t H5T_HDSETREGREF_ALIGN_g = 0;
+size_t H5T_REF_ALIGN_g         = 0;
 
-/*
- * Alignment constraints for native types. These are initialized at run time
- * in H5Tinit.c
- */
 size_t H5T_NATIVE_SCHAR_ALIGN_g   = 0;
 size_t H5T_NATIVE_UCHAR_ALIGN_g   = 0;
 size_t H5T_NATIVE_SHORT_ALIGN_g   = 0;
@@ -777,6 +763,9 @@ H5T_init(void)
     if (H5T__init_native() < 0)
         HGOTO_ERROR(H5E_DATATYPE, H5E_CANTINIT, FAIL, "unable to initialize interface")
 
+    if (H5T__init_native_int() < 0)
+        HGOTO_ERROR(H5E_DATATYPE, H5E_CANTINIT, FAIL, "unable to initialize integers")
+
     /* Get the atomic datatype structures needed by the initialization code below */
     if (NULL == (native_schar = (H5T_t *)H5I_object(H5T_NATIVE_SCHAR_g)))
         HGOTO_ERROR(H5E_ARGS, H5E_BADTYPE, FAIL, "not a datatype object")