Simplify autoconf checking
Simplify autoconf checking

Fold some checks already implicit in the libtool one and require C99
instead of checking for standard headers and libraries

mstorsjo/vo-amrwbenc,jiangjianping/vo-amrwbenc,Distrotech/vo-amrwbenc,BelledonneCommunications/vo-amrwbenc,BelledonneCommunications/vo-amrwbenc,mstorsjo/vo-amrwbenc,jiangjianping/vo-amrwbenc,Distrotech/vo-amrwbenc,Distrotech/vo-amrwbenc