Merge branch 'framework/fix_pio_lib_checks_v7.0' into hotfix-v7.1 (PR #914)

This merge updates the logic that is used to add the PIO library to the
definition of LIBS in the top-level Makefile so that the libpio.settings file
found in newer PIO 2 releases (beginning around PIO 2.5.2) is ignored. Without
the changes in this merge, builds that reference the latest PIO library will
fail with the message

  Checking for a usable PIO library...
  ************ ERROR ************
  Failed to compile a PIO test program
  Please ensure the PIO environment variable is set to the PIO installation directory
  ************ ERROR ************

Note: This merge contains identical changes to PR #707, but this merge is based
off of the v7.0 tag and targets the hotfix-v7.1 branch.

Author: mgduda

Makefile

@@ -1,4 +1,4 @@
-MODEL_FORMULATION = 
+MODEL_FORMULATION =
 
 
 dummy:
@@ -421,9 +421,9 @@ llvm:
 	"OPENMP = $(OPENMP)" \
 	"CPPFLAGS = $(MODEL_FORMULATION) -D_MPI" )
 
-CPPINCLUDES = 
-FCINCLUDES = 
-LIBS = 
+CPPINCLUDES =
+FCINCLUDES =
+LIBS =
 
 #
 # If user has indicated a PIO2 library, define USE_PIO2 pre-processor macro
@@ -458,9 +458,15 @@ endif
 # Depending on PIO version, libraries may be libpio.a, or libpiof.a and libpioc.a
 # Keep open the possibility of shared libraries in future with, e.g., .so suffix
 #
+# Check if libpio.* exists and link -lpio if so, but we make an exception for
+# libpio.settings (a file added in PIO2), which is not a library to link
 ifneq ($(wildcard $(PIO_LIB)/libpio\.*), )
-	LIBS += -lpio
+	# Makefiles don't support "and" operators so we have nested "if" instead
+	ifneq "$(wildcard $(PIO_LIB)/libpio\.*)" "$(PIO_LIB)/libpio.settings"
+		LIBS += -lpio
+	endif
 endif
+
 ifneq ($(wildcard $(PIO_LIB)/libpiof\.*), )
 	LIBS += -lpiof
 endif