编译freeswitch，错误如下：
In file included from libs/spandsp/src/plc.c:42:0: libs/spandsp/src/floating_fudge.h: In function ‘sinf’: libs/spandsp/src/floating_fudge.h:40:20: error: implicit declaration of function ‘sin’ [-Werror=implicit-function-declaration] return (float) sin((double) x);
这个时候如果纠结于这个错误，是无法解决的。正确办法是：
cd spandsp ./configure checking for TIFFOpen in -ltiff... no configure: error: "Cannot build without libtiff (does your system require a libtiff-devel package?)"
原来是缺少这个包。安装即可。
