Android: tell libtiff where libm is

[jolavillette]

build_tiff fails at configure time, and takes build_cimg and build_phash
down with it:

-- Looking for pow
-- Looking for pow - not found
-- Looking for pow
-- Looking for pow - not found
CMake Error at FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find CMath (missing: CMath_pow)
Call Stack (most recent call first):
  cmake/FindCMath.cmake:51 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:156 (find_package)

tiff's cmake/FindCMath.cmake probes pow(3) twice — once with no extra
library, then again with whatever find_library(NAMES m) returned:

check_symbol_exists(pow "math.h" CMath_HAVE_LIBC_POW)
find_library(CMath_LIBRARY NAMES m)

if(NOT CMath_HAVE_LIBC_POW)
    set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${CMath_LIBRARY})
    check_symbol_exists(pow "math.h" CMath_HAVE_LIBM_POW)

On Android pow is in libm, and the NDK keeps libm.so under the API level
directory of the sysroot — sysroot/usr/lib/<triple>/<api>/ — which
andro_cmake does not add to CMAKE_LIBRARY_PATH; it sets it to ${PREFIX}/lib
only. So find_library returns NOTFOUND, CMAKE_REQUIRED_LIBRARIES gains
nothing, the second probe links exactly like the first one, and both fail with
undefined symbol: pow. Note the two identical Looking for pow - not found
lines in the output above: that pair is the signature of this bug.

Passing CMath_LIBRARY explicitly fixes it. The alternative — adding the API
level directory to the library search path in andro_cmake — would change how
the ten dependencies that already build resolve their libraries, for one
consumer.

Verified

build_tiff, build_cimg and build_phash all return 0, and the whole AAR now
builds: 42 MB, jni/arm64-v8a/libretroshare.so exporting JNI_OnLoad, the four
org.retroshare.service classes in classes.jar. Ubuntu 24.04, NDK
29.0.14206865, API level 24, arm64-v8a.