The documentation disclaims support for building with a C++ compiler, so
it is reasonable to assume that the library will be built with a plain C
compiler, so the functions will all have plain C linkage.  By default, a
C++ application that wished to use libiniparser would need to wrap the
inclusion of libiniparser headers in 'extern "C" { ... }' to reflect the
C linkage of libiniparser.  Instead, place that marker directly in the
libiniparser headers, so that client applications do not need to care.
This has no effect on normal compilation of libiniparser, since the new
markers are inside a '#ifdef __cplusplus' guard, and straight C
compilers do not define __cplusplus.

Switch from always hiding the executed command to hide it only by
default, but let the user request that the commands be shown.  For now,
follow the convention that setting $V requests a verbose compilation.

LDSHFLAGS specifies -rpath, but is only used when LDFLAGS are specified,
and LDFLAGS also set the same -rpath.  Remove the redundant definition
in LDSHFLAGS.

The default definition from GNU make is more comprehensive.  Switch to
that, and allow an environmental override.

If the user has set a value for CC, AR, or RM, use that value.  GNU make
provides good defaults for all of these.

If the user has set CFLAGS, use that instead of -O2, but preserve the
other build options.

Append the local LDFLAGS to any value from the user.

Function to obtain the number and list of keys in a section.

Add ability to dump a single section from a dictionary to a loadable ini file.

malloc fix to enable compilation with g++

g++ returns an error when a non-void pointer is set to be the return
value from malloc.