Added GNU compile flags to enable -fPIC for llvm (#4553)

Currently, running ./configure with --enable-fpic=yes and --comp=llvm
does not add the `-fPIC` flag.
This PR adds the -fPIC flag to c++ and fortran compile flags when using
GNUMake and LLVM.

Author: bsrunnels

Tools/GNUMake/comps/gnu.mak

@@ -133,8 +133,8 @@ endif
 
 
 ifeq ($(USE_COMPILE_PIC),TRUE)
-  CXXFLAGS = -fPIC
-  CFLAGS = -fPIC
+  CXXFLAGS += -fPIC
+  CFLAGS += -fPIC
 endif
 
 ifeq ($(ERROR_DEPRECATED),TRUE)
@@ -195,8 +195,8 @@ endif
 
 ifeq ($(USE_COMPILE_PIC),TRUE)
 
-  FFLAGS = -fPIC
-  F90FLAGS = -fPIC
+  FFLAGS += -fPIC
+  F90FLAGS += -fPIC
 
 endif
 

Tools/GNUMake/comps/llvm.mak

@@ -59,6 +59,11 @@ ifeq ($(WARN_ERROR),TRUE)
   CFLAGS += -Werror
 endif
 
+ifeq ($(USE_COMPILE_PIC),TRUE)
+  CXXFLAGS += -fPIC
+  CFLAGS += -fPIC
+endif
+
 # disable some warnings
 CXXFLAGS += -Wno-c++17-extensions
 
@@ -73,6 +78,14 @@ endif
 CXXFLAGS += -std=$(CXXSTD)
 CFLAGS   += -std=c11
 
+
+ifeq ($(USE_COMPILE_PIC),TRUE)
+
+  FFLAGS += -fPIC
+  F90FLAGS += -fPIC
+
+endif
+
 FFLAGS   += -ffixed-line-length-none -fno-range-check -fno-second-underscore
 F90FLAGS += -ffree-line-length-none -fno-range-check -fno-second-underscore -fimplicit-none