Makefile.am: Use $(CXXCOMPILE) to compile C++ code

When compiling the SWIG wrappers for the Java bindings, use the correct
automake macro so that all compiler flags are honoured.
C++: Provide fallbacks for missing stoi/stod functions

Notably, Android does not provide these functions.  The fallback
implementation is based on the one in the GNU ISO C++ Library.
build: Silence java-clean.
C++: Add sanity checks in StructureWrapper.
error.c: Add missing entries.
unit tests: Update to recent API changes.
agilent-dmm: U124x/U125x fixes.
gmc-mh-1x-2x-rs232: Completed energy measurement ranges (V, A, W) for Metrahit 29S.
agilent-dmm: U124xx/U125xx: support 5 more modes.

This adds support for resistance, capacitance, frequency, continuity,
and temperature.
agilent-dmm: Add temperature regex to U125xx parser.
input/wav: Fix broken handling of float32 samples on big endian

Also, make sure that floats are 32 bit even in the case of an
extensible header.
input/wav: Correctly check supported sample size.
scpi/usbtmc: Minor debug output changes.
C++: Remove erroneous stray method.
C++: Fix duplicated shared_ptr creation.
agilent-dmm: U124xx/U125xx: Add current loop sensor support.

Also fixes frequency support.
input/wav: Now really correctly check supported sample size.
C++: Don't spell out arguments to StructureWrapper when not required.
C++: Make StructureWrapper a recurring template, eliminating lots of casts.
C++: Rename StructureWrapper to ParentOwned.