libA
+ CMakeLists.txt
+ mySRC/headerA.h
+ mySRC/headerB.h
+ mySRC/moduleA/modA.h
+ mySRC/moduleB/modB.h

SET(Source_dir mySRC)
SET(LIB_NAME libA)
...
INSTALL(TARGETS ${LIB_NAME} DESTINATION /usr/local/lib)
INSTALL(DIRECTORY ${Source_dir}
DESTINATION /usr/local/include/libA
FILES_MATCHING PATTERN "*.h")

/usr/local/lib/libA.a (which is fine)

/usr/local/include/libA/mySRC/*
/usr/local/include/libA/mySRC/moduleA
/usr/local/include/libA/mySRC/moduleB

/usr/local/include/libA/*
/usr/local/include/libA/moduleA
/usr/local/include/libA/moduleB