Overhaul Make makefiles

* Header files and correspondent C sources that define
    functions recently added to public API moved from
    'tools' directory to root

  * By default, install prefix is '/usr/local/', and header
    install path is '${PREFIX}/include/kissfft'

  * Added detection of external libraries like FFTW or libPNG

Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>

kfc.h

@@ -0,0 +1,54 @@
+/*
+ *  Copyright (c) 2003-2004, Mark Borgerding. All rights reserved.
+ *  This file is part of KISS FFT - https://github.com/mborgerding/kissfft
+ *
+ *  SPDX-License-Identifier: BSD-3-Clause
+ *  See COPYING file for more information.
+ */
+
+#ifndef KFC_H
+#define KFC_H
+#include "kiss_fft.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+KFC -- Kiss FFT Cache
+
+Not needing to deal with kiss_fft_alloc and a config 
+object may be handy for a lot of programs.
+
+KFC uses the underlying KISS FFT functions, but caches the config object. 
+The first time kfc_fft or kfc_ifft for a given FFT size, the cfg 
+object is created for it.  All subsequent calls use the cached 
+configuration object.
+
+NOTE:
+You should probably not use this if your program will be using a lot 
+of various sizes of FFTs.  There is a linear search through the
+cached objects.  If you are only using one or two FFT sizes, this
+will be negligible. Otherwise, you may want to use another method 
+of managing the cfg objects.
+ 
+ There is no automated cleanup of the cached objects.  This could lead 
+to large memory usage in a program that uses a lot of *DIFFERENT* 
+sized FFTs.  If you want to force all cached cfg objects to be freed,
+call kfc_cleanup.
+ 
+ */
+
+/*forward complex FFT */
+void KISS_FFT_API kfc_fft(int nfft, const kiss_fft_cpx * fin,kiss_fft_cpx * fout);
+/*reverse complex FFT */
+void KISS_FFT_API kfc_ifft(int nfft, const kiss_fft_cpx * fin,kiss_fft_cpx * fout);
+
+/*free all cached objects*/
+void KISS_FFT_API kfc_cleanup(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif