Support pkg-config in ta-lib for CMake Builds

[vikasnkumar]

The autoconfbuild supports pkg-config file generation and install, but the CMake build did not. Adding a couple of lines in CMake to support this on UNIX platforms.

[mario4tier]

Merged, thanks! Followed up with 8cf0839 on one detail in ta-lib.pc.in.

The added Libs: ... -lm wasn't quite right: since #77, libta-lib.so already self-declares its libm dependency (DT_NEEDED libm.so.6), so a dynamic consumer needs nothing extra — that's what Libs.private: is for (only pulled in for static linking via pkg-config --static). The actual gap was that Libs.private: @LIBM@ relies on autoconf's AC_SUBST([LIBM]), which CMake's configure_file has no equivalent for, so @LIBM@ was silently substituting to empty on the CMake path.

Follow-up gives CMake its own LIBM value (same if(NOT WIN32) idiom already used to link the target) and feeds that into Libs.private instead, so Libs: stays clean and matches autotools' output:

Libs: -L${libdir} -lta-lib
Libs.private: -lm

Verified with an actual CMake configure+build that the .so still carries DT_NEEDED libm.so.6 and the generated .pc now matches the above.

[vikasnkumar]

Thank you !