Define a cpp macro specific to BLIS compilation.

Details:
- Tweaked the cflags functions in common.mk so that a new preprocessor
  macro, BLIS_IS_BUILDING_LIBRARY, is defined, but only when BLIS
  itself is being built. This macro will not be defined when, for
  example, the testsuite or example code compiles code local to those
  applications. This was done in part by defining a new cflags function
  get-user-cflags-for(), which is now the designated function for
  application Makefiles if they wish to inherit a basic set of CFLAGS
  from BLIS. (The compiler flags returned are identical to that of
  get-frame-cflags-for() except that -DBLIS_IS_BUILDING_LIBRARY is
  omitted.)
- Updated all test driver-like makefiles to call get-user-cflags-for()
  instead of get-frame-cflags-for().

test/exec_sizes/Makefile

@@ -132,12 +132,12 @@ TEST_OBJ_PATH  := .
 TEST_SIZES_SRC := test_size.c
 
 # Override the value of CINCFLAGS so that the value of CFLAGS returned by
-# get-frame-cflags-for() is not cluttered up with include paths needed only
+# get-user-cflags-for() is not cluttered up with include paths needed only
 # while building BLIS.
 CINCFLAGS      := -I$(INC_PATH)
 
 # Use the CFLAGS for the configuration family.
-CFLAGS         := $(call get-frame-cflags-for,$(CONFIG_NAME))
+CFLAGS         := $(call get-user-cflags-for,$(CONFIG_NAME))
 
 # Add local header paths to CFLAGS
 CFLAGS         += -I$(TEST_SRC_PATH)