Fix handling of libtirpc pkg-config files with -L flags

[Bo98]

When querying the libtirpc pkg-config, TIRPC_LIBS may return a value that includes a -L flag which messes up the sed that's performed here. It could technically return multiple -l flags as well.

Example of it breaking:

  checking for xdr_void in -l-L/home/linuxbrew/.linuxbrew/Cellar/libtirpc/1.3.3/lib -ltirpc... no
  checking for xdr_uint32_t in -l-L/home/linuxbrew/.linuxbrew/Cellar/libtirpc/1.3.3/lib -ltirpc... no
  checking for xdr_u_int32_t in -l-L/home/linuxbrew/.linuxbrew/Cellar/libtirpc/1.3.3/lib -ltirpc... no
  checking for xdr_uint in -l-L/home/linuxbrew/.linuxbrew/Cellar/libtirpc/1.3.3/lib -ltirpc... no
  checking for xdr_u_int in -l-L/home/linuxbrew/.linuxbrew/Cellar/libtirpc/1.3.3/lib -ltirpc... no

Instead, we can simplify this by getting rid of the sed and skipping the AC_CHECK_LIB checks by simply hardwiring libtirpc to choose the integer functions that all supported libtirpc versions are known to have.

[jgallagher59701]

I'll run this so that it can access AWS.