modified jansson.pc generating releated Cmake var from path to string

[shouhuanxiaoji]

modified type of JANSSON_INSTALL_LIB_DIR/JANSSON_INSTALL_BIN_DIR/JANSSON_INSTALL_INCLUDE_DIR to string

if they were path, there will be a awkward situation:
I expect to set the setting of .pc to (for Fedora rpm package) :

prefix=/usr
exec_prefix=${prefix}
libdir=${exec_prefix}/lib64
includedir=${prefix}/include

Name: Jansson
Description: Library for encoding, decoding and manipulating JSON data
Version: 2.13.1
Libs: -L${libdir} -ljansson
Cflags: -I${includedir}

but we cannot set JANSSON_INSTALL_LIB_DIR to "lib64" because its type is path, so I will get like "${exec_prefix}//builddir/build/SOURCES/jansson-2.13.1/build/lib64"