CMake: #include <math.h> and link against -lm when checking if lrint is available

The check for lrint in cmake\OpusConfig.cmake doesn't do its job properly and ends up silently dumping the following error into cmake errors.txt. Libopusfile does this check correctly though.
Determining if the function lrint exists failed with the following output:
Change Dir: /mnt/c/py/miniaudio/_skbuild/linux-x86_64-3.8/cmake-build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_939d2 && [1/2] Building C object CMakeFiles/cmTC_939d2.dir/CheckFunctionExists.c.o
: warning: conflicting types for built-in function ‘lrint’; expected ‘long int(double)’ [-Wbuiltin-declaration-mismatch]
/usr/share/cmake-3.16/Modules/CheckFunctionExists.c:7:3: note: in expansion of macro ‘CHECK_FUNCTION_EXISTS’
7 | CHECK_FUNCTION_EXISTS(void);
| ^~~~~~~~~~~~~~~~~~~~~
/usr/share/cmake-3.16/Modules/CheckFunctionExists.c:1:1: note: ‘lrint’ is declared in header ‘<math.h>’
+++ |+#include <math.h>
1 | #ifdef CHECK_FUNCTION_EXISTS
[2/2] Linking C executable cmTC_939d2
FAILED: cmTC_939d2
: && /usr/bin/cc -DCHECK_FUNCTION_EXISTS=lrint -rdynamic CMakeFiles/cmTC_939d2.dir/CheckFunctionExists.c.o -o cmTC_939d2 && :
/usr/bin/ld: CMakeFiles/cmTC_939d2.dir/CheckFunctionExists.c.o: in function main': CheckFunctionExists.c:(.text+0x14): undefined reference to lrint'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.

Inspect cmake\OpusConfig.cmake and compare its lrint availability check with the corresponding libopusfile check mentioned in the issue. Reproduce the CMake configuration check, then verify that the completed check accounts for <math.h> and libm so the reported linker failure no longer occurs.